board: atmel: Fix compiler warning 'board_usb_hw_init()' not used

Wrap definition of board_usb_hw_init() around with #ifdef CONFIG_CMD_USB
to avoid warning: 'board_usb_hw_init' defined but not used
[-Wunused-function] when compiling without CONFIG_CMD_USB.

This patch makes sama5d27_som1_ek, sama5d2_ptc_ek and sama5d2_xplained
consistent with other boards that use the same #ifdef to avoid the warning.

Cc: Eugen Hristev <eugen.hristev@microchip.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Josef Lusticky <josef@lusticky.cz>
Reviewed-by: Eugen Hristev <eugen.hristev@microchip.com>