arch: riscv: introduce RISCV_PRIVILEGED

Introduce a new arch level Kconfig option to signal the implementation
of the RISCV Privileged ISA spec. This replaces
SOC_FAMILY_RISCV_PRIVILEGED, because this is not a SoC specific
property, nor a SoC family.

Note that the SoC family naming scheme will be fixed in upcoming
commits.

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