hw/misc/bcm2835_cprman: add a clock mux skeleton implementation

The clock multiplexers are the last clock stage in the CPRMAN. Each mux
outputs one clock signal that goes out of the CPRMAN to the SoC
peripherals.

Each mux has at most 10 sources. The sources 0 to 3 are common to all
muxes. They are:
   0. ground (no clock signal)
   1. the main oscillator (xosc)
   2. "test debug 0" clock
   3. "test debug 1" clock

Test debug 0 and 1 are actual clock muxes that can be used as sources to
other muxes (for debug purpose).

Sources 4 to 9 are mux specific and can be unpopulated (grounded). Those
sources are fed by the PLL channels outputs.

One corner case exists for DSI0E and DSI0P muxes. They have their source
number 4 connected to an intermediate multiplexer that can select
between PLLA-DSI0 and PLLD-DSI0 channel. This multiplexer is called
DSI0HSCK and is not a clock mux as such. It is really a simple mux from
the hardware point of view (see https://elinux.org/The_Undocumented_Pi).
This mux is not implemented in this commit.

Note that there is some muxes for which sources are unknown (because of
a lack of documentation). For those cases all the sources are connected
to ground in this implementation.

Each clock mux output is exported by the CPRMAN at the qdev level,
adding the suffix '-out' to the mux name to form the output clock name.
(E.g. the 'uart' mux sees its output exported as 'uart-out' at the
CPRMAN level.)

Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-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>

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