drivers: sdhci-brcmstb: set CQE timer clock frequency

CQHCI keeps track of tags in flight with internal timers, so the clock
frequency driving the timer needs to be specified. The config registers
default to 0 (100kHz) which means timeouts will be significantly shorter
than they should be. Assume the timer clock comes from the controller
base clock.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>