hw/arm/armv7m: Handle disconnected clock inputs

hw/arm/armv7m: Handle disconnected clock inputs

In the armv7m object, handle clock inputs that aren't connected.
This is always an error for 'cpuclk'. For 'refclk' it is OK for this
to be disconnected, but we need to handle it by not trying to connect
a sourceless-clock to the systick device.

This fixes a bug where on the mps2-an521 and similar boards (which
do not have a refclk) the systick device incorrectly reset with
SYST_CSR.CLKSOURCE 0 ("use refclk") rather than 1 ("use CPU clock").

Cc: qemu-stable@nongnu.org
Reported-by: Richard Petri <git@rpls.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220208171643.3486277-1-peter.maydell@linaro.org