zephyr: doc: add a note about alternate function GPIOs
zephyr: doc: add a note about alternate function GPIOs

On NPCX platforms some pins default to a specific (non GPIO) function
after reset, but are switched to GPIO in the initialization code. This
may be confusing if one is only referencing the manual default pin
multiplexing configuration.

Adding a note about that in the GPIO doc page to reference this
behavior.

BRANCH=none
BUG=b:241734677
TEST=check the page on gitiles

Signed-off-by: Fabio Baltieri <1c55e39c05d9ab42261288cf330256b2af6ba179@google.com>
Change-Id: I3f6fa9f93c339771e15ac5b62c53442b28d1d2ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3817775
Reviewed-by: Madhu P <91daf3dcb52497aaee2bfb2e83426acf7b0e812c@google.com>
Reviewed-by: Al Semjonovs <c3a2ecb550cad1e7d5a943ca6241377f7b43ffc4@google.com>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec