ARM: rmobile: Deduplicate R-Car Gen3/Gen4 reset_cpu()

ARM: rmobile: Deduplicate R-Car Gen3/Gen4 reset_cpu()

The reset_cpu() implementation is basically the same across Gen3
SoCs and identical across Gen4 SoCs. Introduce weak default for
reset_cpu(), so that it does not have to be duplicated in every
board file again.

There is a slight difference for CA53 only systems, like E3 and D3,
which now check MIDR for CPU ID first just like the other systems,
but this is OK since the MIDR always returns CA53 core type and the
correct reset register is written.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>