[MIPS] Fix computation of {PGD,PMD,PTE}_T_LOG2.

For the generation of asm-offset.h to work these need to be evaulatable
by gcc as a constant expression.  This issue did exist for a while but
didn't bite because they're only in asm-offset.h for debugging purposes.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>