arch/arm/Makefile: sort machine names

The comment immediately above the block of machine directory names implored
users to keep the macro names sorted, but, alas, this is no longer the case.

Put the list of machine directory names back in alphanumerical order by
CONFIG_* macro name, as the comment asks. Sort all the symbols as though they
all followed the convention of having the string "ARCH_" in the middle of
their definitions.

Signed-off-by: Trevor Woerner <twoerner@gmail.com>