MIPS: kernel: branch: Do not emulate the branch likelies on MIPS R6

MIPS R6 removed the BLTZL, BGEZL, BLTZAL, BGEZAL, BEQL, BNEL, BLEZL,
BGTZL branch likely instructions so we must not try to emulate them on
MIPS R6 if the R2-to-R6 emulator is not present.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>