usb: gadget: at91_udc: mention proper dependency

While building allmodconfig on avr32 the build failed with the error:
"at91_pmc_base" [drivers/usb/gadget/udc/atmel_usba_udc.ko] undefined!

On checking the code it turned out that if CONFIG_OF is defined then it
is using at91_pmc_read() which is using at91_pmc_base. And unless
COMMON_CLK_AT91 is defined we donot have at91_pmc_base. And
COMMON_CLK_AT91 is available with AT91 architecture.
Mention the dependency such that this driver builds with avr32 only if
OF is not enabled.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>