arm64: renesas: Split R-Car Gen3 and Gen4 common board code

arm64: renesas: Split R-Car Gen3 and Gen4 common board code

Split common board code for R-Car Gen3 and Gen4 into separate files.
The R-Car Gen3 board code contains fixups specific to TFA which are
no longer required on R-Car Gen4, keep those fixups in its own file
so they would not interfere with Gen4.

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