riscv: Rework Andes PLMT as a UCLASS_TIMER driver

riscv: Rework Andes PLMT as a UCLASS_TIMER driver

This converts the PLMT driver from the riscv-specific timer interface to be
a DM-based UCLASS_TIMER driver.

The clock-frequency/clocks properties are preferred over timebase-frequency
for two reasons. First, properties which affect a device should be located
near its binding in the device tree. Using timebase-frequency only really
makes sense when the cpu itself is the timer device. This is the case when
we read the time from a CSR, but not when there is a separate device.
Second, it lets the device use the clock subsystem which adds flexibility.
If the device is configured for a different clock speed, the timer can
adjust itself.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>