riscv: stack PMP: fix CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT=y case

Let's honor CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT even for kernel
stacks. This saves one global PMP slot when creating the guard area for
the IRQ stack, and some hw implementations might require that anyway.

With this changes, arch_mem_domain_max_partitions_get() becomes much
more reliable and tests/kernel/mem_protect is more likely to pass even
with the stack guard enabled.

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