hw/arm/bcm2836: Hardcode correct CPU type

hw/arm/bcm2836: Hardcode correct CPU type

Now we have separate types for BCM2386 and BCM2387, we might as well
just hard-code the CPU type they use rather than having it passed
through as an object property. This then lets us put the initialization
of the CPU object in init rather than realize.

Note that this change means that it's no longer possible on
the command line to use -cpu to ask for a different kind of
CPU than the SoC supports. This was never a supported thing to
do anyway; we were just not sanity-checking the command line.

This does require us to only build the bcm2837 object on
TARGET_AARCH64 configs, since otherwise it won't instantiate
due to the missing cortex-a53 device and "make check" will fail.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Andrew Baumann <Andrew.Baumann@microsoft.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180313153458.26822-9-peter.maydell@linaro.org