riscv: Add a SYSCON driver for Andestech's PLMT

riscv: Add a SYSCON driver for Andestech's PLMT

The platform-Level Machine Timer (PLMT) block
holds memory-mapped mtime register associated
with timer tick.

This driver implements the riscv_get_time() which
is required by the generic RISC-V timer driver.

Signed-off-by: Rick Chen <[email protected]>
Cc: Greentime Hu <[email protected]>
Reviewed-by: Bin Meng <[email protected]>
Reviewed-by: Lukas Auer <[email protected]>