arm64: dts: uniphier: Enable spi node for PXs3 reference board

PXs3 reference board has 2 spi connectors. This enables spi0 and spi1,
and adds aliases properties for each spi to determine device name
assignments.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>