pinctrl: generic: Add output-enable property

Add output-enable generic pin configuration property.
This properties allows enabling/disabling pin's output capabilities
without actually driving any value on the line.

Change-Id: If2318eef61d23085b0d58d278e77b6c4500e6747
Acked-by: Rob Herring <robh@kernel.org>
[Added inline elaborations on buffer enabling/disabling]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Git-commit: 425562429d4f3b134c7390249c23a3f647aad199
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>