arch: aarch32: define macro for PendSV IRQ priority level

We introduce a macro to define the IRQ priority level for
PendsV, and use it in arch/arm/include/aarch32/exc.h
to set the PendSV IRQ level. The commit does not change
the behavior of PendSV interrupt.

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