riscv: Allow SOC to override arch_irq_{lock,unlock,unlocked}

RISC-V has a modular design. Some hardware with a custom interrupt
controller needs a bit more work to lock / unlock IRQs.

Account for this hardware by introducing a set of new
z_soc_irq_* functions that can override the default behaviour.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>