meson: gxbb: increase CONFIG_SYS_BOOTM_LEN

A feature rich Linux kernel needs more than 8 MiB.
Hence enlarge CONFIG_SYS_BOOTM_LEN to 64 MiB for the GXBB systems.
As all known GXBB systems have at least 512 MiB of RAM this poses no problem.

Cc: Andreas Färber <afaerber@suse.de>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>