gpioled: use hw pin inversion if available

Add hint.gpioled.%d.invmode to allow setting the pin inversion method. Accept
the following values:

• auto Use hardware pin inversion if available, else fallback to software pin inversion.
• hw Use hardware pin inversion.
• sw Use software pin inversion.

Default is auto. This hint is ignored when no pin inversion is
requested (hint.gpioled.%d.invert=0).

Signed-off-by: Stéphane Rochoy <stephane.rochoy@stormshield.eu>
Sponsored-by: Stormshield
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1805

(cherry picked from commit 9ae3c3010a0deb80eb96b1184734904ed61fecef)