hw/arm/spitz: Provide usual QOM macros for corgi-ssp and spitz-lcdtg

hw/arm/spitz: Provide usual QOM macros for corgi-ssp and spitz-lcdtg

The QOM types "spitz-lcdtg" and "corgi-ssp" are missing the
usual QOM TYPE and casting macros; provide and use them.

In particular, we can safely use the QOM cast macros instead of
FROM_SSI_SLAVE() because in both cases the 'ssidev' field of
the instance state struct is the first field in it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20200628142429.17111-17-peter.maydell@linaro.org