Documentation: dt: soc: Add description for knav qmss driver

Add documentation for knav qmss driver.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>