[ARM] pxa: separate PXA25x and PXA27x UDC register definitions

The PXA25x and PXA27x USB device controller register definitions are
different.  Currently, they live side by side in pxa-regs.h, but only
one set is available depending on the setting of PXA25x or PXA27x.

This means that if we build to support both PXA25x and PXA27x, the
PXA27x definitions are unavailable, even to PXA27x specific code.

Remove these definitions from pxa-regs.h, and place them in separate
files.  Include these files where appropriate.

Note: according to the dependencies in drivers/usb/gadget/Kconfig,
we do not support the UDC on PXA27x nor PXA3xx CPUs, so remove the
platform devices from pxa27x.c and pxa3xx.c.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>