dt-bindings: pinctr: pinctrl-zynq: Fix compatible string

dt-bindings: pinctr: pinctrl-zynq: Fix compatible string

Compatible string doesn't really match with compatible string listed in the
driver itself. While binding was converted from txt to yaml
xlnx,zynq-pinctrl was listed as compatible string but example was using
xlnx,pinctrl-zynq and also this string is used in all DTSes.
xlnx,zynq-pinctrl is used only in dt binding and not present in any DT
which is stable for quite a long time that's why use old compatible string
and update binding document instead of starting to use unused compatible
string.

Fixes: 153df45acda0 ("dt-bindings: pinctrl: pinctrl-zynq: Convert to yaml")
Signed-off-by: Michal Simek <michal.simek@amd.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/c1307a4dd5e30290acacc786cb2170deb9eaa539.1706087258.git.michal.simek@amd.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>