arm64: dts: rockchip: Add basic cpu frequencies for RK3368

This adds and enable the operating points that have been tested and are
currently supported by the SoC. This also adds clocks for ARMCLKL and
ARMCLKB.

Signed-off-by: Romain Perier <romain.perier@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>