ARM: uniphier: make SPL optional for ARVv8 SoCs

We may want to run different firmware before running U-Boot.  For
example, ARM Trusted Firmware runs before U-Boot, making U-Boot
a non-secure world boot loader.  In this case, the SoC might be
initialized there, which enables us to skip SPL entirely.

This commit removes "select SPL" to make it configurable.  This
also enables the Multi SoC support for the UniPhier ARMv8 SoCs.
(CONFIG_ARCH_UNIPHIER_V8_MULTI)  Thanks to the driver model and
Device Tree, the U-Boot proper part is now written in a generic way.
The board/SoC parameters reside in DT.  The Multi SoC support
increases the memory footprint a bit, but the U-Boot proper does
not have strict memory constraint.  This will mitigate the per-SoC
(sometimes per-board) defconfig burden.

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