Add definitions for EntryLo register bits in mipsregs.h. The R4000
compatible ones are prefixed MIPS_ENTRYLO_ and the R3000 compatible ones
are prefixed R3K_ENTRYLO_.

These will be used in later patches.
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Maciej W. Rozycki <macro@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/10073/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>