ARM: rmobile: Convert R8A7791 Porter board to DM and DT

Convert the R8A7791 Porter board to DM and DT. This implies mostly
enabling the necessary configuration options and plucking out the
ad-hoc configuration from the board file. Moreover, the pre-reloc
malloc size was increased to allow the clock driver to start up
early without running out of malloc space and the early stack was
moved further up in the DRAM to avoid rewriting U-Boot itself.

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