of: of_test: add MODULE_DESCRIPTION()

Fix the 'make W=1' warning:
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/of/of_test.o

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Link: https://lore.kernel.org/r/20240524-md-of-of_test-v1-1-6ebd078d620f@quicinc.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>