ARM: rmobile: Limit bootloader size to 1 MiB on R-Car Gen3

The HF/QSPI flash layout permits up to 1 MiB large bootloader blob,
set CONFIG_BOARD_SIZE_LIMIT to enforce this limit and set the
monitor size to match accordingly.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>