hw/misc/bcm2835_cprman: add a PLL channel skeleton implementation

hw/misc/bcm2835_cprman: add a PLL channel skeleton implementation

PLLs are composed of multiple channels. Each channel outputs one clock
signal. They are modeled as one device taking the PLL generated clock as
input, and outputting a new clock.

A channel shares the CM register with its parent PLL, and has its own
A2W_CTRL register. A write to the CM register will trigger an update of
the PLL and all its channels, while a write to an A2W_CTRL channel
register will update the required channel only.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Luc Michel <luc@lmichel.fr>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>