arch: riscv: smp: allow other IPI implementation

The currently IPI implementation assumes that CLINT exists in the
system, however, that might not be the case as IPI can be implemented
with PLIC that supports software-triggering as well, such as the Andes
NCEPLIC100.

Refactor the CLINT-based IPI implementations into `ipi_clint.c`, and
create Kconfig that selects the CLINT implementation when
`sifive-clint0` exists and enabled, otherwise default to
`RISCV_SMP_IPI_CUSTOM` which allows OOT implementation. This also
makes way for the upstreaming of non-clint IPI implementation later.

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