MIPS: Allow MIPS_CPU_SCACHE to be used with different line sizes
CONFIG_MIPS_CPU_SCACHE determines whether to build sc-mips.c. However,
it is currently hardwired to use an L1_SHIFT of 6 (64 bytes). Move the
L1_SHIFT selection into the CPU or SoC section so that other SoCs can
select different values.