ARM: dts: rockchip: Fix vcc5/6-supply representation on rv1108-elgin

On rv1108-elgin-r1 board the RK805 VCC5 and VCC6 supplies come from
the BUCK2 regulator at 2.2V, so fix the representation in the
device tree.

While at it, rename it from vdd_cam to vdd_buck2, which is a better
name for the regulator label.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>