hw/misc/bcm2835_cprman: implement PLLs behaviour

hw/misc/bcm2835_cprman: implement PLLs behaviour

The CPRMAN PLLs generate a clock based on a prescaler, a multiplier and
a divider. The prescaler doubles the parent (xosc) frequency, then the
multiplier/divider are applied. The multiplier has an integer and a
fractional part.

This commit also implements the CPRMAN CM_LOCK register. This register
reports which PLL is currently locked. We consider a PLL has being
locked as soon as it is enabled (on real hardware, there is a delay
after turning a PLL on, for it to stabilize).

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>