ARM: dts: sun6i: a31s: bpi-m2: improve pmic properties

The eldoin is supplied from the dcdc1 regulator. The N_VBUSEN pin is
connected to an external power regulator (SY6280AAC).
With this commit we update the pmic binding properties to support
those features.

Fixes: 7daa2137 ("ARM: dts: sunxi: Add regulators for Sinovoip BPI-M2")
Cc: <stable@vger.kernel.org>
Signed-off-by: Philipp Rossak <embed3d@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>