spl: mmc: Add option to set eMMC HW boot partition

This change allows setting pre-defined eMMC boot partition for SPL eMMC
booting. It is necessary in the case when one wants to boot (through falcon
boot) from eMMC after loading SPL from other memory (like SPI-NOR).

Signed-off-by: Mans Rullgard <mans@mansr.com>
[lukma: Edit the commit message]
Signed-off-by: Lukasz Majewski <lukma@denx.de>
Acked-by: Andreas Dannenberg <dannenberg@ti.com>