ARM: 8541/1: decompressor: drop redundant FORCE in Makefile

The object "piggy.$(suffix_y).o" is created from "piggy.$(suffix).S"
by the following pattern rule defined in scripts/Makefile.build:

  $(obj)/%.o: $(src)/%.S FORCE
          $(call if_changed_dep,as_o_S)

FORCE is already added to the prerequisite of the object there.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>