MIPS: Detect bad _PFN_SHIFT values

2 recent commits have fixed issues where _PFN_SHIFT grew too large due
to the introduction of too many pgprot bits in our PTEs for some MIPS32
kernel configurations. Tracking down such issues can be tricky, so add a
BUILD_BUG_ON() to help.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org