riscv: rationalize PMP related Kconfig options

ARCH_HAS_USERSPACE and ARCH_HAS_STACK_PROTECTION are direct functions
of RISCV_PMP regardless of the SoC.

PMP_STACK_GUARD is a function of HW_STACK_PROTECTION (from
ARCH_HAS_STACK_PROTECTION) and not the other way around.

This allows for tests/kernel/fatal/exception to test protection against
various stack overflows based on the PMP stack guard functionality.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>