pinctrl: zte: fix group_desc initialization

There are a couple of issues with group_desc initialization in function
zx_pinctrl_build_state().

 - num_pins is not initialized and remains zero.
 - pins shouldn't be initialized with a pointer to variable in the
   stack.

With them fixed, pin_request() in pinmux_enable_setting() can be invoked
correctly.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>