ssi: cache SSIPeripheralClass to avoid GET_CLASS()

Investigating why some BMC models are so slow compared to a plain ARM
virt machines I did some profiling of:

  ./qemu-system-arm -M romulus-bmc -nic user \
    -drive
    file=obmc-phosphor-image-romulus.static.mtd,format=raw,if=mtd \
    -nographic -serial mon:stdio

And saw that object_class_dynamic_cast_assert was dominating the
profile times. We have a number of cases in this model of the SSI bus.
As the class is static once the object is created we just cache it and
use it instead of the dynamic case macros.

Profiling against:

  ./tests/venv/bin/avocado run \
    tests/avocado/machine_aspeed.py:test_arm_ast2500_romulus_openbmc_v2_9_0

Before: 35.565 s ±  0.087 s
After: 15.713 s ±  0.287 s

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Cédric Le Goater <clg@kaod.org>
Tested-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20220811151413.3350684-6-alex.bennee@linaro.org>
Message-Id: <20220923084803.498337-6-clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

include/hw/ssi/ssi.h

@@ -59,6 +59,9 @@ struct SSIPeripheralClass {
 struct SSIPeripheral {
     DeviceState parent_obj;
 
+    /* cache the class */
+    SSIPeripheralClass *spc;
+
     /* Chip select state */
     bool cs;
 };