ARM: shmobile: rcar-gen2: Use ICRAM1 for jump stub on all SoCs

Currently the different SoCs in the R-Car Gen2 family use different
types of on-chip RAM for the jump stub:
  - R-Car H2 uses Media RAM,
  - R-Car M2-W uses another type of optional On-chip RAM, as it doesn't
    have Media RAM,
  - R-Car M2-N uses Inter Connect RAM in Magnus Damm's "ARM: shmobile:
    r8a7793 boot address update".

As all R-Car Gen2 SoCs have 4 KiB of Inter Connect RAM, consolidate the
code by always using that.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>