riscv: aclint_ipi: Support T-Head C900 CLINT

riscv: aclint_ipi: Support T-Head C900 CLINT

Although timer component of the CLINT isn't fully compatible with the
generic RISC-V ACLINT, the IPI component behaves the same.

As the CLINT doesn't have corresponding riscv_aclint_timer driver
available, let's try looking for a compatible SYSCON device directly
when no riscv_aclint_timer device could be found on IPI initialization.

Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>