MIPS: move create_tlb() in an proper header: mipsregs.h

Export create_tlb() as an inline function in mipsregs.h. It allows to
remove the declaration of the function from the board files.

Then it will allow also to use this function very early in the boot when
the stack is not usable.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>