clocksource/drivers/timer-mediatek: Implement CPUXGPT timers

clocksource/drivers/timer-mediatek: Implement CPUXGPT timers

Some MediaTek platforms with a buggy TrustZone ATF firmware will not
initialize the AArch64 System Timer correctly: in these cases, the
System Timer address is correctly programmed, as well as the CNTFRQ_EL0
register (reading 13MHz, as it should be), but the assigned hardware
timers are never started before (or after) booting Linux.

In this condition, any call to function get_cycles() will be returning
zero, as CNTVCT_EL0 will always read zero.

One common critical symptom of that is trying to use the udelay()
function (calling __delay()), which executes the following loop:

            start = get_cycles();
            while ((get_cycles() - start) < cycles)
                    cpu_relax();

which, when CNTVCT_EL0 always reads zero, translates to:

            while((0 - 0) < 0)  ==> while(0 < 0)

... generating an infinite loop, even though zero is never less
than zero, but always equal to it (this has to be researched,
but it's out of the scope of this commit).

To fix this issue on the affected MediaTek platforms, the solution
is to simply start the timers that are designed to be System Timer(s).
These timers, downstream, are called "CPUXGPT" and there is one
timer per CPU core; luckily, it is not necessary to set a start bit
on each CPUX General Purpose Timer, but it's conveniently enough to:
 - Set the clock divider (input = 26MHz, divider = 2, output = 13MHz);
 - Set the ENABLE bit on a global register (starts all CPUX timers).

The only small hurdle with this setup is that it's all done through
the MCUSYS wrapper, where it is needed, for each read or write, to
select a register address (by writing it to an index register) and
then to perform any R/W on a "CON" register.

For example, writing "0x1" to the CPUXGPT register offset 0x4:
- Write 0x4 to mcusys INDEX register
- Write 0x1 to mcusys CON register

Reading from CPUXGPT register offset 0x4:
- Write 0x4 to mcusys INDEX register
- Read mcusys CON register.

Finally, starting this timer makes platforms affected by this issue
to work correctly.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://lore.kernel.org/r/20220613133819.35318-3-angelogioacchino.delregno@collabora.com
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>