ARM: UniPhier: optimize kicking secondary CPUs code

Currently, the secondary CPU(s) are kicked three times:
Boot ROM ---(kick)--> SPL ---(kick)--> U-boot ---(kick)--> Linux.
It makes the boot sequence very complicated.

This commit merges the first and the second kicks, so the secondary
CPU(s) can directly jump from SPL to Linux.
arch/arm/mach-uniphier/smp.S is no longer necessary.

Linux boot test passed.

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