am43xx_evm: Add DFU support for qspi flash

This adds support to update firmware on qspi flash present on
am437x-sk-evm and am43xx-epos-evm via DFU.

On device:
=> setenv dfu_alt_info ${dfu_alt_info_qspi}
=> dfu 0 sf 0:0

On host:
$ sudo dfu-util -l
$ sudo dfu-util -D u-boot.bin -a u-boot.bin

Signed-off-by: Vignesh R <vigneshr@ti.com>