arm: socfpga: sockit: Probe DWC2 UDC from OF instead of hard-coded data

This patch adds the necessary OF alias for the UDC node, which let's
the code locate the DWC2 UDC base address in OF instead of hard-coding
it into the U-Boot binary. The code is adjusted to use the address from
OF instead of the hard-coded one. Finally, the hard-coded address is
removed and USB DM support is enabled.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Lukasz Majewski <l.majewski@majess.pl>
Cc: Lukasz Majewski <l.majewski@samsung.com>