hw/misc/zynq_slcr: add clock generation for uarts

hw/misc/zynq_slcr: add clock generation for uarts

Add some clocks to zynq_slcr
+ the main input clock (ps_clk)
+ the reference clock outputs for each uart (uart0 & 1)

This commit also transitional the slcr to multi-phase reset as it is
required to initialize the clocks correctly.

The clock frequencies are computed using the internal pll & uart configuration
registers and the input ps_clk frequency.

Signed-off-by: Damien Hedde <damien.hedde@greensocs.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20200406135251.157596-7-damien.hedde@greensocs.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>