arm: dts: rk3399-rockpro64: slightly increase center voltage

The rk3399 VD_CENTER voltage domain is not subject to dynamic voltage
scaling. So the regulator reset voltage of 0.9V is used on this board.
Let u-boot initialize the center voltage to 0.95V as it is done for the
VD_LOGIC domain. This avoids instability and occasional linux kernel
Opses on this board.

Signed-off-by: Soeren Moch <smoch@web.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>