splash_source: add support for ubifs formatted nand

Add support for loading splash image from NAND Flash formatted with a (UBI) filesystem.

Signed-off-by: Eran Matityahu <eran.m@variscite.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Tom Rini <trini@konsulko.com>
Cc: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Stefano Babic <sbabic@denx.de>
Acked-by: Nikita Kiryanov <nikita@compulab.co.il>