staging: vchiq: provide testing instructions

This small document should to help kernel developer to test their
changes on the VCHIQ driver.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Link: https://lore.kernel.org/r/20220618090002.20663-1-stefan.wahren@i2se.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>