soc: riscv: privilege: define soc_interrupt_init() as weak symbol

Define soc_interrupt_init as a weak symbol in the common RISC-V
privileged instruction set SoC support.

This allows overriding soc_interrupt_init for SoCs which are not fully
compliant with the RISC-V privileged specification.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>