MIPS: bcm63xx: cpu: Replace BUG() with panic()

MIPS: bcm63xx: cpu: Replace BUG() with panic()

BUG() can be a noop if CONFIG_BUG is not selected,
leading to the following build problem on a randconfig:

arch/mips/bcm63xx/cpu.c: In function 'detect_cpu_clock':
arch/mips/bcm63xx/cpu.c:254:1: error: control reaches end of
non-void function [-Werror=return-type]

We fix this problem by replacing BUG() with panic() since it's
best to handle the case of an unknown board instead of silently
returning a random clock frequency.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Acked-by: Steven J. Hill <Steven.Hill@imgtec.com>
Acked-by: Jonas Gorski <jogo@openwrt.org>
Acked-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5932/