aspeed/scu: Introduce a aspeed_scu_get_apb_freq() routine

The APB frequency can be calculated directly when needed from the
HPLL_PARAM and CLK_SEL register values. This removes useless state in
the model.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20190904070506.1052-11-clg@kaod.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>