pinctrl: aspeed-g6: Add support for the AST2600 USB pinmux

pinctrl: aspeed-g6: Add support for the AST2600 USB pinmux

AST2600 has two USB ports, A, B:

Port A supports 4 distinct modes:
	1. PCIe EHCI to Hub
	2. Hub to PHY
	3. BMC EHCI to PHY
	4. PCIe EHCI to PHY

Port B support 3 modes:
	1. USB1.1 HID controller
	2. USB2.0 Device controller
	3. BMC EHCI port2

Implement pinmux support by mapping each ports' functions onto a single
pin group for each port.

Signed-off-by: Johnny Huang <johnny_huang@aspeedtech.com>
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20191202061432.3996-4-andrew@aj.id.au
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>