spl: add support for custom boot method names

Currently the names MMC1, MMC2 and MMC2_2 are output in the SPL. To
achieve more userbility here the name of the boot source can be returned.
E.g. for "MMC1" -> "eMMC" or "MMC2" -> "SD card".

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Tested-by: Michael Walle <michael@walle.cc>