hw/ssi: imx_spi: Rework imx_spi_reset() to keep CONREG register value

When the block is disabled, all registers are reset with the
exception of the ECSPI_CONREG. It is initialized to zero
when the instance is created.

Ref: i.MX 6DQ Applications Processor Reference Manual (IMX6DQRM),
     chapter 21.7.3: Control Register (ECSPIx_CONREG)

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210129132323.30946-5-bmeng.cn@gmail.com
[bmeng: add a 'common_reset' function that does most of reset operation]
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>