pinctrl: set unit for debounce time pinconfig to usec

Currently the debounce time pinconfig option uses an unspecified
"time units" unit. As pinconfig options should use SI units and a
real unit is also necessary for generic dt bindings, change it
to usec. Currently no driver is using the generic pinconfig option
for this, so the unit change is safe to do.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>