We already have some PBL MCI implementations in barebox, but none
are used for chainloading a barebox from a file system.

There are some SoCs that would benefit from this however:
At least the Zynq, AT91, SoCFPGA and TI SoCs.

In preparation for supporting first stage boot on these where it's
customary for both the BootROM and first stage bootloader to load the
follow-up stage from FAT, add a very basic block I/O API that MCI
drivers can implement.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>