board: gateworks: venice: dynamically determine U-Boot raw sector

board: gateworks: venice: dynamically determine U-Boot raw sector

CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR needs to adjust for
IMX8MN and IMX8MP when booting from an eMMC boot partition due
to IMX BOOTROM v2 using an SPL offset of 0 for boot partitions
and 32K for the user partition.

In order to allow the same firmware to run on both user and boot
hardware partitions adjust raw_sect dynamically at runtime.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>