drivers: intc: plic: fix for SMP=n when MP_MAX_NUM_CPUS > 1

The functions to obtain the address are hardcoded to return
the address of the first core when `CONFIG_SMP != y`, this
causes an issue with enabling an IRQ when there are more than
one core in the system (`CONFIG_MP_MAX_NUM_CPUS > 1`), as the
driver would first enable the IRQ on the first core, and when
it tries to obtain the address for the following cores and
disable the IRQ on them, the functions continue to return the
address of the first core, causing the IRQ to be disabled
on the first core.

Fix this by determine if `CONFIG_MP_MAX_NUM_CPUS > 1` instead
of `CONFIG_SMP=y` when returning the address.

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