mx6sabresd: Reduce U-Boot proper size to fix boot regression

mx6sabresd: Reduce U-Boot proper size to fix boot regression

The mx6sabresd U-Boot proper binary size has grown beyond the
CONFIG_BOARD_SIZE_LIMIT.

Reduce its size by removing the CONFIG_MULTI_DTB_FIT, BOOTM_PLAN9 and
BOOTM_RTEMS options.

According to doc/README.multi-dtb-fit:

"Usually the DTB is selected by the SPL and passed down to U-Boot. But some
platforms don't use the SPL. In this case MULTI_DTB_FIT can used to provide
U-Boot with a choice of DTBs"

mx6sabresd uses SPL, so MULTI_DTB_FIT can be safely dropped as the DTB
selection in SPL is done by board_fit_config_name_match().

Tested boot on the imx6dl and imx6q variants.

Signed-off-by: Fabio Estevam <festevam@gmail.com>