octeontx2-af: Support masking of PFC classes

commit 941f2accfdc86fb943f7d54940aaf2a329b77f7b from
git@git.assembla.com:cavium/WindRiver.linux.git

Both Octeontx2 MAC (CGX) and CN10K MAC (RPM) supports masking of
priority flow control classes this gives privilege to user that
only few classes can be enabled. This patch configures the
necessary registers to support the same.

Other changes:-
On CN10k PFC_MODE needs to be cleared to generate 802.3 pause frames.
Currently this bit is not cleared when user disables PFC. This patch
fixes this issue.

Fixes: 8c880f8c94ec ("octeontx2-af: Priority flow control configuration support")
Change-Id: Ie97cc93515b554139677238c2b38c4dd099bfe63
Signed-off-by: Hariprasad Kelam <hkelam@marvell.com>
Reviewed-on: https://sj1git1.cavium.com/c/IP/SW/kernel/linux/+/70456
Tested-by: sa_ip-sw-jenkins <sa_ip-sw-jenkins@marvell.com>
Reviewed-by: Sunil Kovvuri Goutham <sgoutham@marvell.com>
Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>