serial: Finish sysrq on qcom_geni; fix sysrq vs. lockdep on 8250

I started out this series trying to make sysrq work over the serial console on qcom_geni_serial, then fell into a rat's nest.

To solve the deadlock I faced when enabling sysrq I tried to borrow code from '8250_port.c' which avoided grabbing the port lock in console_write(). ...but since these days I try to run with lockdep on all the time, I found it caused an annoying lockdep splat (which I also reproduced on my rk3399 board). ...so I instead changed my qcom_geni_serial solution to borrow code from 'msm_serial.c'

I wasn't super happy with the solution in 'msm_serial.c' though. I don't like releasing the spinlock there. Not only is it ugly but it means we are unlocking / re-locking _all the time_ even though sysrq characters are rare. ...so I came up with what I think is a better solution and then implemented it for qcom_geni_serial.

Since I had a good way to test 8250-based UARTs, I also fixed that driver to use my new method. When doing so, I ran into a missing include in serial_core.h. NOTE: I didn't have a way to test msm_serial.c at all, so I didn't switch that (or all other serial drivers for that matter) to the new method.

NOTE: from a serial point of view v2 is the same as v1 but I've removed the extra kgdb-related patches and made it obvious that this is really for all sysrq, not just kgdb. I've also generally tried to curate the CCs more properly.