configs: Add phycore_am62x_r5_usbdfu_defconfig

This config includes the phycore_am62x_r5_defconfig file as well as
the am62x_r5_usbdfu.config fragment. We need another defconfig
because the AM62x has not enough internal SRAM to support all boot
sources. The normal phycore_am62x_r5_defconfig should allow to boot
from MMC and OSPI while this new defconfig allows to boot from USB.

Signed-off-by: Daniel Schultz <d.schultz@phytec.de>