arm: mach-k3: am6_init: Do USB fixups to facilitate host and device boot modes

arm: mach-k3: am6_init: Do USB fixups to facilitate host and device boot modes

U-boot only supports either USB host or device mode for a node at a
time in dts. To support both host and dfu bootmodes, set "peripheral"
as the default dr_mode but fixup property to "host" if host bootmode
is detected.

This needs to happen before the dwc3 generic layer binds the usb device
to a host or device driver. Therefore, add an fdtdec_setup_board()
implementation to fixup the dt based on the boot mode.

Also use the same fixup function to set the USB-PCIe Serdes mux to PCIe
in both the host and device cases. This is required for accessing the
interface at USB 2.0 speeds.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>