MIPS: update MIPS_L1_CACHE_SHIFT based on MIPS_L1_CACHE_SHIFT_<N>

All platforms that require a special MIPS_L1_CACHE_SHIFT value have been
updated, such that we can now make MIPS_L1_CACHE_SHIFT default to the
appropriate integer value based on the select MIPS_L1_CACHE_SHIFT_<N>
variable.

Signed-off-by: Florian Fainelli <florian@openwrt.org>