MIPS: Make CONFIG_SYS_DCACHE_LINE_SIZE int, not hex

For consistency with the other cache-related Kconfig entries & the
values actually set by boards, make CONFIG_SYS_DCACHE_LINE_SIZE an int
entry instead of a hex entry.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Fixes: 37228621 ("MIPS: Split I & D cache line size config")