board: gateworks: venice: increase CONFIG_SYS_SPL_MALLOC_SIZE

commit 03f1f78a ("spl: fit: Prefer a malloc()'d buffer for loading images")'
changed the way buffer allocation worked for SPL to a more flexible
method.

For venice this caused breakage that is resolved by increasing the size
of CONFIG_SYS_SPL_MALLOC_SIZE as the current FIT slighly exceeds 512KiB.

Additionally remove the unnecessary comment on CONFIG_SPL_BSS_MAX_SIZE
and CONFIG_SYS_SPL_MALLOC_SIZE as the size is obvious from the define.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>