soc: ti: k3-navss-ringacc: Remove unused ring modes

With AM64x supporting only K3_NAV_RINGACC_RING_MODE_RING or the exposed
ring mode, all other K3 SoCs have also been moved to this common
baseline. Therefore drop other modes such as
K3_NAV_RINGACC_RING_MODE_MESSAGE (and proxy) to save on SPL footprint.

There is a saving of ~800 bytes with this change for am65x_evm_r5_defconfig.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>