gpio: pxa: fix Documentation of interrupt-names property

The property name to pass names for interrupt resources is called
"interrupt-names", not "interrupt-name". Fix it to avoid further
confusion.

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>