spi: orion: yield to scheduler in transfer wait loop

orion_spi_wait_till_ready() busy-waits in a tight udelay(1) loop, up to
2000 iterations, and is called per byte from the polled, byte-at-a-time
transfer path. On SoCs such as the Armada 388 (e.g. SolidRun Helios4),
which also run SATA over the shared internal MBus fabric, this stalls
the CPU for the whole transfer and delays servicing of SATA interrupts.
Under sustained activity this can cause SATA timeouts and link resets
(sometimes renegotiating down to SATA II, 3 Gbps).

Add cond_resched() to the wait loop so the scheduler can run pending
IRQs between polls. This is a no-op at runtime unless the kernel is
built with CONFIG_PREEMPT enabled, where it lets other peripheral
interrupts be serviced during SPI transfers.

Built with LLVM=1 ARCH=powerpc; passes checkpatch --strict.

Assisted-by: opencode:big-pickle
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://patch.msgid.link/20260907005750.230103-1-rosenp@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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