ARM: uniphier: use Image.gz instead Image for booting ARM64 Linux

ARM: uniphier: use Image.gz instead Image for booting ARM64 Linux

The ARM64 Linux raw image now amounts to 15MB and it is getting
bigger and bigger.  Using Image.gz saves about 8MB.  The cost of
unzip is smaller than what we get by saving the kernel loading
from non-volatile devices.

The ARM32 Linux still uses zImage, a self-decompressor image,
so it should not be affected.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>