ARM: rmobile: Fix SD divider settings on Gen3

On RCar M3 and on RCar H3 newer than and not including ES1.0, the SD clock
must be divided by 4 rather than 2 because a hardware workaround present
only in the H3 ES1.0 has been removed from these chips. U-Boot currently
only supports M3 and H3 ES 2.0 and newer, so configure the SD pre-divider
to 4 to prevent SD instability.

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