boards: bt510, b610: move some gpio-keys entries zephyr,user

These two boards have few oddly defined gpio-keys nodes that do not seem
to have anything to do with keys, and are just used as a placeholder to
define gpios.

Switch the gpio entries to regulators where appropriate, move the two
mag sensors to their own node, drop the rest.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>