configs: am62x_evm_*: Fix USB DFU configuration

configs: am62x_evm_*: Fix USB DFU configuration

CONFIG_USB_XHCI_DWC3 is not required for AM62x as the XHCI
driver is registered through the dwc3-generic driver.

CONFIG_USB_XHCI_DWC3 causes problems by hijacking the
USB controller even if it is not set for Host mode in
device tree.

'dm tree' output after 'usb start' is fixed from

 simple_bus    5  [ + ]   dwc3-am62             |   |-- dwc3-usb@f900000
 usb_gadget    0  [   ]   dwc3-generic-periphe  |   |   |-- usb@31000000
 usb           0  [ + ]   xhci-dwc3             |   |   `-- usb@31000000
 usb_hub       0  [ + ]   usb_hub               |   |       `-- usb_hub
 simple_bus    6  [ + ]   dwc3-am62             |   |-- dwc3-usb@f910000
 usb           1  [ + ]   dwc3-generic-host     |   |   |-- usb@31100000
 usb_hub       1  [ + ]   usb_hub               |   |   |   `-- usb_hub
 usb           1  [ + ]   xhci-dwc3             |   |   `-- usb@31100000
 usb_hub       2  [ + ]   usb_hub               |   |       `-- usb_hub

[notice that 'xhci-dwc3' and 'usb_hub' drivers are probed
 for both USB instances although the first instance
 is supposed to be 'peripheral' only]

to

 simple_bus    5  [   ]   dwc3-am62             |   |-- dwc3-usb@f900000
 usb_gadget    0  [   ]   dwc3-generic-periphe  |   |   `-- usb@31000000
 simple_bus    6  [ + ]   dwc3-am62             |   |-- dwc3-usb@f910000
 usb           1  [ + ]   dwc3-generic-host     |   |   `-- usb@31100000
 usb_hub       0  [ + ]   usb_hub               |   |       `-- usb_hub

Fixes: dfc2dff5a844 ("configs: am62x_evm_*: Enable USB and DFU support")
Signed-off-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Siddharth Vadapalli <s-vadapalli@ti.com>