s3adsp1800: Remove the hardcoded values from the reset

Remove the hardcoded values from the machine specific reset
function, as the same values are already set in the standard
MicroBlaze reset.

This also allows the entire reset function to be deleted, as
PVR registers are now preserved on reset.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>