[rpi] Add fdt_pinctrl(4) support to Raspberry Pi GPIO driver

On Raspberry Pi platform GPIO controller also responsible for pins
multiplexing. Pi code predates proper FDT support in FreeBSD so a
lot of pinmux info is hardcoded. This patch:

• Implements pinctl methods in bcm2835_gpio
• Converts all devices with ad-hoc pinmux info to proper pin control mechanisms and adds pinmux info in FreeBSD's custom dts files.
• Adds fdt_pinctrl option to RPI2 and RPI-B kernels
• Adds SPI pinmux config to FreeBSD's customization of GNU DTS.

Reviewed by: imp, manu
Differential Revision: https://reviews.freebsd.org/D14104