[POWERPC] of/gpio: Use dynamic base allocation

Since the "gpiolib: dynamic gpio number allocation" patch was recently
merged into Linus' tree (commit 8d0aab2f16c4fa170f32e7a74a52cd0122bbafef),
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>