configs: am62x_evm_*: Enable USB and DFU support

Provide config fragments to enable USB host as well as USB gadget and DFU
support for a53 and r5. This relevant fragment is included into the
am62x EVM a53 defconfig. For the r5, due to the smaller available size,
the config fragment also disables support for persistent storage to free
up space for USB support. This fragment needs to be included is DFU
booting is desired.

The CONFIG_DFU_SF option is placed in the defconfig rather than the
fragment as this is known not to be supported on all boards that can
support DFU.

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