riscv: Rework Sifive CLINT as UCLASS_TIMER driver

riscv: Rework Sifive CLINT as UCLASS_TIMER driver

This converts the clint driver from the riscv-specific interface to be a
DM-based UCLASS_TIMER driver. In addition, the SiFive DDR driver previously
implicitly depended on the CLINT to select REGMAP.

Unlike Andes's PLMT/PLIC (which AFAIK never have anything pass it a dtb),
the SiFive CLINT is part of the device tree passed in by qemu. This device
tree doesn't have a clocks or clock-frequency property on clint, so we need
to fall back on the timebase-frequency property. Perhaps in the future we
can get a clock-frequency property added to the qemu dtb.

Unlike with the Andes PLMT, the Sifive CLINT is also an IPI controller.
RISCV_SYSCON_CLINT is retained for this purpose.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Pragnesh Patel <pragnesh.patel@openfive.com>