arm: rmobile: Add support koelsch board

The koelsch board has R8A7791, 2GB DDR3-SDRAM, USB,
Quad SPI, Ethernet, and more.

This patch supports the following functions:
 - DDR3-SDRAM
 - SCIF

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Hisashi Nakamura <hisashi.nakamura.ak@renesas.com>
CC: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
CC: Albert Aribaud <albert.u.boot@aribaud.net>