greybus: db3-platform: fix some typos

Fix misspelled "mandatory," and use "GPIOs" for the plural form (no
apostrophe and capitalized) in comments.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>