dt-bindings: pinctrl: toshiba,visconti: Fix number of items in groups

dt-bindings: pinctrl: toshiba,visconti: Fix number of items in groups

The "groups" property can hold multiple entries (e.g.
toshiba/tmpv7708-rm-mbrc.dts file), so allow that by dropping incorrect
type (pinmux-node.yaml schema already defines that as string-array) and
adding constraints for items.  This fixes dtbs_check warnings like:

  toshiba/tmpv7708-rm-mbrc.dtb: pinctrl@24190000 (toshiba,tmpv7708-pinctrl):
    pwm-pins:groups: ['pwm0_gpio16_grp', 'pwm1_gpio17_grp', 'pwm2_gpio18_grp', 'pwm3_gpio19_grp'] is too long

Fixes: 1825c1fe0057 ("pinctrl: Add DT bindings for Toshiba Visconti TMPV7700 SoC")
Cc: stable@vger.kernel.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>