arm: Cortex-M0: Adapt core register code to M0

The Cortex-M0(+) and in general processors that support only the ARMv6-M
instruction set have a reduced set of registers and fields compared to
the ARMv7-M compliant processors.
This change goes through all core registers and disables or removes
everything that is not part of the ARMv6-M architecture when compiling
for Cortex-M0.

Jira: ZEP-1497

Change-id: I13e2637bb730e69d02f2a5ee687038dc69ad28a8
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>