arch: riscv: introduce a Kconfig to mask `mhartid`

Configure a new Kconfig (`CONFIG_RISCV_HART_MASK`) so that it
is possible to mask the `mhartid` of a processor. This is
helpful in the cases when the hart id starts from non-zero
value.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>