ARM: uniphier: allow to run zImage rather than uImage

UniPhier SoC family adopt ARM Multi-platform in Linux since the first
upstreaming.  Because CONFIG_ARM_PATCH_PHYS_VIRT is defined, the
kernel image is completely position-independent.  There is no reason
to decide the load address on compile time, but it is up to the boot
loader.  Now, zImage is handier than uImage, also it allows to skip
the relocation of the kernel image.

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