soc: ti: k3-ringacc: Provide documentation for 'k3_ring's 'state'

Fixes the following W=1 kernel build warning(s):

 drivers/soc/ti/k3-ringacc.c:163: warning: Function parameter or member 'state' not described in 'k3_ring'

Cc: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>