usb: gadget: at91_udc: Rework for multi-platform kernel support

cpu_is_at91xxx are a set of macros defined in mach/cpu.h and are here used
to detect the SoC we are booting on.
Use compatible string + a caps structure to replace those cpu_is_xxx tests.

Remove all mach and asm headers (which are now unused).

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