beagleplay: Add DFU support

DFU mode on a beagleplay can be used via the Type-C connector by holding
the USR switch while powering on.

Configuration is already provided as fragments for both the A53 and R5
u-boot parts. Include the am62x_a53_usbdfu.config config. The
am62x_r5_usbdfu.config fragment needs to be added should DFU boot be
required as this will disable booting from persistent storage due to
binary size constraints.

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>
Signed-off-by: Martyn Welch <martyn.welch@collabora.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>