ti_armv7_common.h: Fix U-Boot location on eMMC

ti_armv7_common.h: Fix U-Boot location on eMMC

According to common eMMC partition table for Android boot (see
PARTS_DEFAULT definition in include/configs/dra7xx_evm.h), "bootloader"
partition (where u-boot.img is stored) starts at 256 KiB. Which is equal
to 512 sectors (as 1 MMC sector size is 512 bytes).

This patch fixes CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR constant so that
it points to correct address of "bootloader" partition and SPL is able to
read, parse and run u-boot.img correctly.

This change was originally done as part of patch [1] in omapzoom u-boot.
Without this patch, SPL fails to parse U-Boot header with next error:

    mkimage signature not found - ih_magic = 4814325a

While at it, also fix U-Boot partition size, which is 384 KiB (as stated in
include/configs/dra7xx_evm.h).

[1] http://omapzoom.org/?p=repo/u-boot.git;a=commit;h=742b82d0c0aa0ed8096c2225a00e9f350212efa9

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>