MIPS: Malta: Enable BLK_DEV_INITRD

This configuration is useful for boot testing malta_defconfig in QEMU
with just a simple cpio initrd, instead of a full ext4 rootfs.

This results in an increase of ~164KB of vmlinux (with GCC 11.2.0):

$ diskus vmlinux.before
11.19 MB (11,194,368 bytes)

$ diskus vmlinux.after
11.36 MB (11,358,208 bytes)

This size increase comes from the fact that usr/Kconfig is sourced when
CONFIG_BLK_DEV_INITRD is enabled, which defaults to supporting several
decompression algorithms for compressed initrds. This seems like a
reasonable tradeoff but these configurations could be disabled in the
future if there are complaints about the size increase.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>