[MIPS] Kill redundant EXTRA_AFLAGS

Many Makefiles in arch/mips have EXTRA_AFLAGS := $(CFLAGS) line.  This
is redundant while AFLAGS contains $(cflags-y) and any options only
listed in CFLAGS (not in cflags-y) should be unnecessary for asm
sources.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>