MIPS: introduce MIPS_L1_CACHE_SHIFT_<N>

In order to avoid keeping an ever growing list of chips which need to
select a specific MIPS_L1_CACHE_SHIFT value introduce multiple internal
and non-exposed Kconfig symbols for the various MIPS_L1_CACHE_SHIFT
values out there and update the relevant Kconfig symbols to select them.

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