MIPS: BCM63xx: Fix typo in cpu-feature-overrides file.

Fix typo: CONFIG_BCMCPU_IS_63xx does not exist;
CONFIG_BCM63XX_CPU_63xx is the valid config option.

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
To: linux-mips@linux-mips.org
Cc: Maxime Bizon <mbizon@freebox.fr>
Patchwork: http://patchwork.linux-mips.org/patch/901/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>