ARM: rmobile: rcar-gen3: Activate bootm_size

ARM: rmobile: rcar-gen3: Activate bootm_size

v2019.01 commit 07a8060a1277 ("ARM: rmobile: Convert to bootm_size")
attempted converting to bootm_size of 256 MiB below targets:
 - include/configs/alt.h
 - include/configs/gose.h
 - include/configs/koelsch.h
 - include/configs/lager.h
 - include/configs/porter.h
 - include/configs/rcar-gen3-common.h
 - include/configs/silk.h
 - include/configs/stout.h

The update didn't fully work on R-Car3 due to the
CONFIG_SYS_BOOTMAPSZ=8MiB definition left by v2016.09 commit
e525d34b476e ("ARM: rmobile: Add support salvator-x board"),
which is explained in below README excerpt:

----8<----
CONFIG_SYS_BOOTMAPSZ:
                [..] If CONFIG_SYS_BOOTMAPSZ is undefined,
                then the value in "bootm_size" will be used instead.
----8<----

Allow the original commit to accomplish its purpose on R-Car3 targets
by removing the CONFIG_SYS_BOOTMAPSZ definition.

Fixes: 07a8060a1277 ("ARM: rmobile: Convert to bootm_size")
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>