MIPS: mips32/cache.S: remove superfluous register assignment

The t4 register already holds the cache
line size, and the value of the register
is not changed in mips_init_icache.

Get the cache line size value from t4 for
mips_init_dcache as well and remove the
superfluous assignment of t5 register.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>