SPL from nand will print 'NAND' in boot_from_devices based on
the image_loader name, remove the extra 'NAND ' in mxs_nand_spl driver.

Original behaviour:
-------------------
U-Boot SPL 2017.01-rc2-gf84dd8b (Jan 02 2017 - 22:24:19)
Trying to boot from NANDNAND : 512 MiB

After the fix:
-------------
U-Boot SPL 2017.01-rc2-gf84dd8b-dirty (Jan 02 2017 - 23:17:00)
Trying to boot from NAND: 512 MiB

Cc: Tom Rini <trini@konsulko.com>
Signed-off-by: Jagan Teki <jagan@openedev.com>