ARM: rmobile: Convert Porter to SPL

Due to size limitations of the MERAM, switch U-Boot to SPL.
The SPL is loaded by the SPI_LOADER into MERAM and then loads
U-Boot proper into DRAM. This way U-Boot can freely grow in
size in DRAM, as there is plenty of it.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
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NOTE: To update U-Boot, first install u-boot.img to 0x140000 in SPI NOR,
      then use the Minimon to flash u-boot-spl.srec using ls,2,e6304000.
      To generate u-boot-spl.srec, use objcopy:
        arm-linux-gnueabi-objcopy -O srec spl/u-boot-spl u-boot-spl.srec