dt-bindings: Make AXP20X compatible strings one per line

In the binding documentation of AXP20X mfd, the compatible strings used
to be listed for three per line, which leads to some mess when trying to
add AXP803 compatible string (as we have already AXP806 and AXP809
compatibles, which is after AXP803 in ascending order).

Make the compatible strings one per line, so that inserting a new
compatible string will be directly a new line.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Lee Jones <lee.jones@linaro.org>