arm: am57xx: Increase "bootloader" partition size

Increase the size of u-boot.img, so that it conforms with new DFU
configuration (see commit [1]).

[1] 7a53a1a8 ARM: ti: Update layout for MMC and eMMC (env and dfu)

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