[MIPS] Make two functions static

The following routines

uasm_rel_highest()
uasm_rel_higher()

are needlessly defined global. This patch makes them static.

Compile-tested using a customized config for the Malta board. Booting
the same board up to the shell prompt was also successful with this
patch applied.

Spotted by sparse.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.fi>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>