spi: dw: use threaded interrupt

Jisheng Zhang <jszhang@kernel.org> says:

To avoid blocking for an excessive amount of time, eventually impacting
on system responsiveness, hard interrupt handlers should finish
executing in as little time as possible.

Use threaded interrupt and move the SPI transfer handling to an
interrupt thread when non-native CS and host only.

After that, since the dw_reader() and dw_writer() are called in
threaded ISR now, so we can delay the unmasking interrupts until no
rx and tx action is taken, thus reduce the interrupt numbers further.

Tested with below two cmds
./spidev_test -D /dev/spidev1.3 -s 30000000 -S 327680 -I 1

./spidev_test -D /dev/spidev1.3 -s 30000000 -S 327680 -I 1000
./rtla timerlat top -q -k -P f:95

The first cmd is to check the interrupt numbers optmizaion result, the
2nd cmd group is to check the threaded interrupt improvement.

Before the patch:
each 320KB spi spidev_test transfer triggers 33118 interrupts

spidev_test reports ~22090 kbps
and rtla reports:
                                     Timer Latency
  0 00:00:37   |          IRQ Timer Latency (us)        |         Thread Timer Latency (us)
CPU COUNT      |      cur       min       avg       max |      cur       min       avg       max
  0 #9958      |        1         0        67    103394 |        6         4      2198    105031
  1 #36902     |        1         0         1        18 |        5         4         5        29

After the patch:
each 320KB spi spidev_test transfer only triggers 1 interrupts
spidev_test reports ~23520 kbps
and now rtla reports:
                                    Timer Latency
  0 00:00:58   |          IRQ Timer Latency (us)        |         Thread Timer Latency (us)
CPU COUNT      |      cur       min       avg       max |      cur       min       avg       max
  0 #58362     |        1         0         0        29 |        6         3         4        56
  1 #58363     |        1         0         1        23 |        6         4         5        68

In summary:
before the patch	after the patch
33118 interrutps	1 interrupts		reduced by 33117 times!
103394 us max latency	29 us max latency	reduced by 3564 times!
22090 kbps rate		23520 kbps rate		improved by 6.5%

Link: https://patch.msgid.link/20261005151739.25233-1-jszhang@kernel.org

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