verdin-am62: add u-boot update wrappers

Add update_tiboot3, update_tispl and update_uboot wrappers to update
R5 SPL, A53 SPL and A53 U-boot respectively.

Usage example:
> tftpboot ${loadaddr} tiboot3-am62x-gp-verdin.bin
> run update_tiboot3

> tftpboot ${loadaddr} tispl.bin
> run update_tispl

> tftpboot ${loadaddr} u-boot.img
> run update_uboot

Signed-off-by: Emanuele Ghidoli <emanuele.ghidoli@toradex.com>
Acked-by: Francesco Dolcini <francesco.dolcini@toradex.com>