[POWERPC] of/gpio: Use dynamic base allocation

Since the "gpiolib: dynamic gpio number allocation" patch was recently
merged into Linus' tree (commit 8d0aab2f),
we can use dynamic GPIO base allocation now.

This, in turn, removes number of gpios per chip constraint.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>