escc: implement soft reset as described in the datasheet

escc: implement soft reset as described in the datasheet

The software reset differs from a device reset in that it only changes the contents
of specific registers. Remove the code that resets all the registers to zero during
soft reset and implement the default values listed in the table in the "Z85C30 Reset"
section.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210903113223.19551-6-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>