ARM: 8542/1: decompressor: merge piggy.*.S and simplify Makefile

The files piggy.$(suffix).S are similar enough to be merged into a
single file.  This also allows clean up of the Makefile.

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