net: ethernet: ti: am65-cpsw/ethtool: Enable RX HW timestamp only for PTP packets

In the current mechanism of timestamping, am65-cpsw-nuss driver
enables hardware timestamping for all received packets by setting
the TSTAMP_EN bit in CPTS_CONTROL register, which directs the CPTS
module to timestamp all received packets, followed by passing
timestamp via DMA descriptors. This mechanism causes CPSW Port to
Lock up.

To prevent port lock up, don't enable rx packet timestamping by
setting TSTAMP_EN bit in CPTS_CONTROL register. The workaround for
timestamping received packets is to utilize the CPTS Event FIFO
that records timestamps corresponding to certain events. The CPTS
module is configured to generate timestamps for Multicast Ethernet,
UDP/IPv4 and UDP/IPv6 PTP packets.

Update supported hwtstamp_rx_filters values for CPSW's timestamping
capability.

Fixes: b1f66a5b ("net: ethernet: ti: am65-cpsw-nuss: enable packet timestamping support")
Signed-off-by: Chintan Vankar <c-vankar@ti.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>