common: spl: Add spl NVMe boot support

Add support to load the next stage image from an NVMe disk which may
be formatted as an EXT or FAT filesystem.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
[trini: Drop hunk changing disk/part.c as that breaks other users]
Signed-off-by: Tom Rini <trini@konsulko.com>