pinctrl: at91: add drive strength config options

The SAMA5 and SAMA92x5, SAM9G25 and SAM9G35 and others implement the ability to select the output drive strength for pins. The configuration typically allows low, medium and high power modes with predefined current limits. This only reserves 2 bits in the pinctrl config(bits 5 and 6) for drive strength. The 2 bits must be set based on the chip's definition for drive strength.

i.e. the A9s do
00 = high
01 = medium
10 = low
11 = reserved/undefined

the A5 does
00 = low
01 = low
10 = medium
11 = high

Signed-off-by: Marek Roszko <mark.roszko@gmail.com>