arch: riscv: use CONFIG_RISCV_MCAUSE_EXCEPTION_MASK

Instead of custom SOC_MCAUSE_EXP_MASK definition. Note that SoCs
selecting RISCV_PRIVILEGED already used such config indirectly (see
changes in soc_common.h).

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