The VPU is responsible for managing the core clock, usually under
direction from the bcm2835-cpufreq driver but not via the clk-bcm2835
driver. Since the core frequency can change without warning, it is
safer to report the maximum clock rate to users of the core clock -
I2C, SPI and the mini UART - to err on the safe side when calculating
clock divisors.
If the DT node for the clock driver includes a reference to the
firmware node, use the firmware API to query the maximum core clock
instead of reading the divider registers.
Prior to this patch, a "100KHz" I2C bus was sometimes clocked at about
160KHz. In particular, switching to the 4.9 kernel was likely to break
SenseHAT usage on a Pi3.
Signed-off-by: Phil Elwell <phil@raspberrypi.org> Signed-off-by: Tim Gardner <tim.gardner@canonical.com>