pinctrl: bcm2835: Add support for output-low output-high properties

Properties to set initial value of pin output buffer.
This can be useful for configure hardware in overlay files, and in early
boot for checking it states in QA sanity tests.

Signed-off-by: Matheus Castello <matheus@castello.eng.br>
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>