Fix FADT parsing

The (1.0 inherited) separate length fields in the FADT are byte granular.
Further, PM1a/b may have distinct lengths and live in distinct address spaces.
 acpi_tb_convert_fadt() should account for all of these conditions.

Apart from these changes I'm puzzled by the fact that, not just for
acpi_gbl_xpm1{a,b}_enable, acpi_hw_low_level_{read,write}() get an explicit
size passed rather than using the size found in the passed GAS.  What happens
on a platform that defines PM1{a,b} wider than 16 bits?  Of course,
acpi_hw_low_level_{read,write}() at present are entirely un-prepared to deal
with sizes other than 8, 16, or 32, not to speak of a non-zero bit_offset or
access_width...

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Len Brown <lenb@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>