SMM: add defines for APM_CNT register

in the current code, the defines for the APM_CNT (0xb2) register
are duplicated in almost every place where it is used. define those
values in cpu/x86/smm.h, and only include this file.

And while at it, fixup whitespace.

Change-Id: Iae712aff53322acd51e89986c2abf4c794e25484
Signed-off-by: Sven Schnelle <svens@stackframe.org>
Reviewed-on: http://review.coreboot.org/4
Tested-by: build bot (Jenkins)