pinctrl: uniphier: add check if pins are valid

The pinctrl datas of uniphier SoCs are the minimal subsets of kernel's one,
and some tables has no data to save the footprint size.
If the board dts tries to match a pin name on no pins defined SoC,
the footprint magic code causes "Synchronous Abort".

This checks if the 'pins' data is valid,
and if empty, avoids the abort with the warning as follows:

  WARNING at drivers/pinctrl/uniphier/pinctrl-uniphier-core.c:36/uniphier_pinctrl_get_pins_count()!
  pinctrl_select_state_full: pinctrl_config_one: err=-38

Signed-off-by: Dai Okamura <okamura.dai@socionext.com>