[fdt_pinctrl] Add man pages for fdt_pinctrl driver

• Add fdt_pinctrl(4) with general information for the driver
• Add fdt_pinctrl(9) with fdt_pinctrl KPI description

Reviewed by: ian, manu, wblock
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D14235