usb: gadget: atmel: access the PMC using regmap

Use regmap to access the PMC to avoid using at91_pmc_read and
at91_pmc_write.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Felipe Balbi <balbi@ti.com>