ARM: dts: rza2mevb: Upstream Linux requires SDRAM

As upstream Linux does not support XIP (eXecute-In-Place) for ARMv7-A
platforms that are part of the ARCH_MULTIPLATFORM framework, it cannot
run in the 8 MiB of HyperRAM present on the RZ/A2M CPU board.  Hence the
64 MiB of SDRAM on the sub-board needs to be enabled, which has the
following ramifications:
  - SCIF4 connected to the on-board USB-serial can no longer be used as
    the serial console,
  - Instead, SCIF2 is used as the serial console, by connecting a 3.3V
    TTL USB-to-Serial adapter to the CMOS camera connector,
  - The first Ethernet channel can no longer be used,
  - USB Channel 1 loses the overcurrent input signal.

Based on the Linux-4.19 BSP for RZ/A2.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Chris Brandt <chris.brandt@renesas.com>
Link: https://lore.kernel.org/r/20210304094945.4003097-1-geert+renesas@glider.be