arch: arm: minor clean-up in interrupt stack size derivation

CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT is taken into
account when allocating the area for the interrupt stack
using the K_THREAD_STACK_DEFINE macro. Therefore, we can
simplify how the top of the stack is derived during the
initialization of the system, by removing one more
occurrence of CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>