mx6sabresd: Reduce overall SPL size

mx6sabresd: Reduce overall SPL size

Currently the SPL binary is 67 kB, which leaves only 1 kB of free
internal RAM space.

The following options can be safely removed to save some precious
SPL space:

- CONFIG_SPL_FS_EXT4: u-boot-dtb.img is stored in raw sector via dd
command (at offset 69 kB)
- CONFIG_SPL_I2C_SUPPORT: I2C is not used during SPL
- CONFIG_SPL_OS_BOOT: no need to make Falcon mode supported
by default

After this change the SPL binary size gets down to 51 kB.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>