ARM: mstar: Add the external clocks to the base dsti

ARM: mstar: Add the external clocks to the base dsti

All of the currently known MStar/SigmaStar ARMv7 SoCs have an "xtal"
clock input that is usually 24MHz and an "RTC xtal" that is usually 32KHz.

The xtal input has to be connected to something so it's enabled by default.

The MSC313 and MSC313E do not bring the RTC clock input out to the pins
so it's impossible to connect it. The SSC8336 does bring the input
out to the pins but it's not always actually connected to something.

The RTC node needs to always be present because in the future the nodes
for the clock muxes will refer to it even if it's not usable.

The RTC node is disabled by default and should be enabled at the board
level if the RTC input is wired up.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Link: https://lore.kernel.org/r/20210301123542.2800643-3-daniel@0x0f.com'
Signed-off-by: Arnd Bergmann <arnd@arndb.de>