gpio: pxa: remove dead code

Commit f8731174 ("ARM: mmp: add more compatible names in gpio driver")
changed the driver logic to determine the number of available GPIOs from
the compatible string, and hence obsoleted the (undocumented) child
nodes that were previously necessary. However, it left some remainder
which can be safely removed now.

Also, this patch makes pxa_gpio_probe_dt() return the correct value in
case irq_alloc_descs() fails.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Cc: Haojian Zhuang <haojian.zhuang@linaro.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>