pinctrl: add bcm2708 driver

This driver is a verbatim copy of the pinctrl-bcm2835 driver, except for:
* changed 2835 to 2708
* gpio_chip and IRQ part are removed
* Probing function is changed.

Because armctrl sets up the gpio irqs, we use the bcm2708_gpio driver.
This hack is used to be able to support both DT and non-DT builds.

Binding document: brcm,bcm2835-gpio.txt
It's not possible to set trigger type and level flags for IRQs in the DT.

Signed-off-by: Noralf Tronnes <notro@tronnes.org>