hw/misc: zynq_slcr: Correctly compute output clocks in the reset exit phase

hw/misc: zynq_slcr: Correctly compute output clocks in the reset exit phase

As of today, when booting upstream U-Boot for Xilinx Zynq, the UART
does not receive anything. Debugging shows that the UART input clock
frequency is zero which prevents the UART from receiving anything as
per the logic in uart_receive().

From zynq_slcr_reset_exit() comment, it intends to compute output
clocks according to ps_clk and registers. zynq_slcr_compute_clocks()
is called to accomplish the task, inside which device_is_in_reset()
is called to actually make the attempt in vain.

Rework reset_hold() and reset_exit() so that in the reset exit phase,
the logic can really compute output clocks in reset_exit().

With this change, upstream U-Boot boots properly again with:

$ qemu-system-arm -M xilinx-zynq-a9 -m 1G -display none -serial null -serial stdio \
    -device loader,file=u-boot-dtb.bin,addr=0x4000000,cpu-num=0

Fixes: 38867cb7ec90 ("hw/misc/zynq_slcr: add clock generation for uarts")
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 20210901124521.30599-2-bmeng.cn@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>