arch/arm/core: In comments, consistently refer to "Cortex-M".

This directory now handles all of Cortex-M0, Cortex-M3/M4. So, just
consistently use "Cortex-M" (as used by number of files already)
without refering to a particular subarch. Also, consistently (letter
casing) spell it as "Cortex-M". A typo is fixed too.

Change-Id: I42ee09abc9a503381bca4ae437c83a8f48816ebc
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>