spl: spl_mmc: Add option to boot from a MMC partition with offset

This patch introduces the option to boot from a MMC card parition with
an offset. This can be done by using both defines together:

define CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION 1
define CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR    ((160 << 10) / 512)

The example above loads the main U-Boot at offset 160KiB from the MMC
partition 1.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Cc: Dirk Eibach <eibach@gdsys.de>
Cc: Tom Rini <trini@konsulko.com>