Two TXDB_V2 channels are used between Linux and System Manager(SM).
Channel0 for normal TX, Channel 1 for notification completion.
The TXDB_V2 trigger logic is using imx_mu_xcr_rmw which uses
read/modify/update logic.

Note: clear MUB GSR BITs, the MUA side GCR BITs will also got cleared per
hardware design.
Channel0 Linux
read GCR->modify GCR->write GCR->M33 SM->read GSR----->clear GSR
                                                |-(1)-|
Channel1 Linux start in time slot(1)
read GCR->modify GCR->write GCR->M33 SM->read GSR->clear GSR
So Channel1 read GCR will read back the GCR that Channel0 wrote, because
M33 has not finish clear GSR, this means Channel1 GCR writing will
trigger Channel1 and Channel0 interrupt both which is wrong.

Channel0 will be freed(SCMI channel status set to FREE) in M33 SM when
processing the 1st Channel0 interrupt. So when 2nd interrupt trigger
(channel 0/1 trigger together), SM will see a freed Channel0, and report
protocol error.

To address the issue, not using read/modify/update logic, just use
write, because write 0 to GCR will be ignored. And after write MUA GCR,
wait the SM to clear MUB GSR by looping MUA GCR value.

Fixes: 5bfe4067 ("mailbox: imx: support channel type tx doorbell v2")
Reviewed-by: Ranjani Vaidyanathan <ranjani.vaidyanathan@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>