dt-bindings: net: broadcom-bluetooth: Add VBAT and VDDIO supplies

The Broadcom Bluetooth chips have two power inputs, VBAT and VDDIO.
The former provides overall power for the chip, while the latter powers
the I/O pins and buffers.

This patch adds properties for the two so we can describe the power
supply relationships.

Reviewed-by: Rob Herring <robh@kernel.org>
Tested-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>