arch: riscv: make __soc_is_irq optional

It looks like all SoCs in tree check if an exception comes from an IRQ
the same way, so let's provide a common logic by default, still
customizable if the SoC selects RISCV_SOC_ISR_CHECK.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>
7 files changed