rockchip: spi: rk_spi: dynamically select an module input rate

rockchip: spi: rk_spi: dynamically select an module input rate

The original clock/bitrate selection code for the rk_spi driver was a
bit limited, as it always selected a 99MHz input clock rate (which
would allow for a maximum bitrate of 49.5MBit/s), but returned -EINVAL
if a bitrate higher than 48MHz was requested.

To give us better control over the bitrate (i.e. add more operating
points, especially at "higher" bitrate---such as above 9MBit/s), we
try to choose 4x the maximum frequency (clamped to 50MBit) from the
DTS instead of 99MHz... for most use-cases this will yield a frequency
of 198MHz, but is flexible to go beyond this in future configurations.

This also rewrites the check to allow frequencies of up to half the
SPI module rate as bitrates and then clamps to whatever the DTS allows
as a maximum (board-specific) frequency and does away with the -EINVAL
when trying to select a bitrate (for cases that exceeded the hard
limit) and instead consistently clamps to the lower of the hard limit,
the soft limit for the SPI bus (from the DTS) or the soft limit for
the SPI slave device.

This replaces
  "rockchip: spi: rk_spi: select 198MHz input to the SPI module for the RK3399"
  "rockchip: spi: rk_spi: improve clocking code for the RK3399"
from earlier versions of this series.

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