introduce lower_32_bits() macro

The file kernel.h contains the upper_32_bits macro.  This patch adds the
other part, the lower_32_bits macro.  Its first use will be in the driver
for AMD IOMMU.

Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>