ARM: renesas: Set R-Car Gen2 board size limit to 512 kiB

The maximum size of u-boot.img on R-Car Gen2 is 0x80000 or 512 kiB,
set the limit to avoid overflows as new functionality gets pulled in.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Paul Barker <paul.barker.ct@bp.renesas.com>