parisc: remove meaningless ccflags-y in arch/parisc/boot/Makefile

This ccflags-y is never used because arch/parisc/boot/Makefile
only contains objcopy and install targets.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Helge Deller <deller@gmx.de>