board: sama5d2_xplained: Clean up code

board: sama5d2_xplained: Clean up code

Since the introduction of pinctrl and clk driver, and the dts file,
remove unneeded the pin configurations and the clock enabling code.

Signed-off-by: Wenyou Yang <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Reviewed-by: Andreas Bießmann <[email protected]>