rockchip: mmc: handle deprecation of 'clock-freq-min-max'

The 'clock-freq-min-max' property was deprecated in the upstream
(i.e. Linux) DTS bindings in favor of the 'max-frequency' property.

With the latest RK3399 DTSI does no longer include the deprecated
property and the rockchip_dw_mmc driver requiring it to be present,
the driver doesn't bind to the node in the RK3399 DTSI any longer
(thus breaking access to the SD card on the RK3399-Q7 board).

To fix this, we implement a similar logic as in the Linux driver: if
the deprecated property is present, we issue a warning (if DEBUG is
enabled); if it is missing, we require 'max-frequency' to be set and
use it to create a min/max value-pair.

See https://github.com/torvalds/linux/commit/b023030f10573de738bbe8df63d43acab64c9f7b


for the deprecation/matching change in Linux.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>