ARM: shmobile: Include all 4 GiB of memory on Lager DT Ref

Add 2GiB of DRAM at 0x1_8000_0000 to support the full 4GiB
of Lager system memory in case of DT Reference.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>