ARM: lib: Remove CONFIG_ARCH_CPU_INIT dependency

Create a weak-aliased arch_cpu_init, so that we can get rid of CONFIG_ARCH_CPU_INIT
and always call arch_cpu_init.

This way we do not need to define CONFIG_ARCH_CPU_INIT in every board file, since
arch_cpu_init() is supposed to handle common CPU level code.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Stefano Babic <sbabic@denx.de>