power: supply: cros: add support for dedicated port

ChromeOS devices can have one optional dedicated port.
The Dedicated port is unique and similar to the USB PD ports
except that it doesn't support as many properties.

The presence of a dedicated port is determined from whether the
EC's charger port count is equal to 'number of USB PD port' + 1.
The dedicated port ID is always the last valid port ID.

This commit keeps compatibility with Embedded Controllers that do not
support the new EC_CMD_CHARGE_PORT_COUNT command by setting
the number of charger port to be equal to the number of USB PD port
when this command fails.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>