ARM: mmp: rename pxa_register_device

In a multiplatform kernel that includes both pxa and mmp, we get a link
failure from the clash of two pxa_register_device functions.

Rename the one in mach-mmp to mmp_register_device, along with with the
rename of pxa_device_desc.

Acked-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>