pinctrl/nomadik: provide stubs for legacy Nomadik

The compilation of the pinctrl driver failed on the legacy
Nomadik NHK8815 platform because it was not providing the PRCMU
interfaces needed to support the extended alternate functions
used by the ux500 series.

Solve this by providing some stubs for the legacy platform, to
avoid too much #ifdefs in the code per se. Theoretically this
actually allows the Nomadik and Ux500 to have a single kernel
image with support for the PRCM registers on the Ux500 (though
they have incompatible archs, but the spirit is there).

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>