MIPS: BCM63XX: select correct MIPS_L1_CACHE_SHIFT value

Broadcom BCM63xx DSL SoCs have a L1-cache line size of 16 bytes (shift
value of 4) instead of the currently configured 32 bytes L1-cache line
size.
Reported-by: Daniel Gonzalez <dgcbueu@gmail.com>
Signed-off-by: Florian Fainelli <florian@openwrt.org>

arch/mips/Kconfig

@@ -138,6 +138,7 @@ config BCM63XX
 	select SWAP_IO_SPACE
 	select ARCH_REQUIRE_GPIOLIB
 	select HAVE_CLK
+	select MIPS_L1_CACHE_SHIFT_4
 	help
 	 Support for BCM63XX based boards