ARM: uniphier: merge two defconfig files

PH1-Pro5 support and ProXstream2/PH1-LD6b support can coexist in one
image and there is bit more room in SPL to accommodate all of them.
Merge uniphier_pro5_defconfig into uniphier_pxs2_defconfig.

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