arm: nvic: Fix exception priority access on Cortex-M0(+)

The Cortex-M0(+) and in general processors that support only the ARMv6-M
instruction set can only access the NVIC_IPRn registers with word
accesses, and not with byte ones like the Cortex-M3 and onwards. This
patch addresses the issue by modifying the way that _NvicIrqPrioSet()
writes to the IPRn register, using a word access for Cortex-M0(+).
A similar issue is addressed for internal exceptions, this time for the
SHPR registers that are accessed differently on ARMv6-M.

Reference code taken from CMSIS.

Jira: ZEP-1497

Change-id: I08e1bf60b3b70579b42f4ab926ee835c18bb65bb
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>