ARM: 9076/1: boot: remove redundant piggy_data from clean-files

Kbuild cleans up files listed in 'targets'.

'piggy_data' is already added to 'targets' a few lines above.

Adding it to 'clean-files' is redundant.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>