[MIPS] make mdesc and prom_getmdesc() static

Neither the mdesc[] array nor the prom_getmdesc() function need to
be global. This patch makes them static.

Successfully build-tested using default configs for Malta, Atlas
and SEAD boards.

Runtime test successfully performed by booting the Malta 4Kc board
up to the shell prompt.

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