Define gpio constants rather than using enum.
Fix pull-up and pull-down values of gpio.
According to A10 user manual possible pull register
values are 00 Pull-up/down disable, 01 Pull-up, 10 Pull-down.

Approved by: gonzo@
kgdb enhancements!

* document the kgdb -b flag
* better verify what's valid with -b
* add more comprehensive command line help

PR:		kern/175743
Submitted by:	Christoph Mallon <christoph.mallon@gmx.de>
Merge branch 'master' of github.com:freebsd/freebsd into test