spi: s3c64xx: add sleep during transfer

In polling mode, the status register is continuously read to check data
transfer completion. It can cause excessive CPU usage.
To reduce this, we can calculate the transfer time and put the sleep during
transfer.

When test on ExynosAuto9 SADK board, throughput remained the same, but
100% CPU utilization decreased to 40%.

Signed-off-by: Jaewon Kim <jaewon02.kim@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230502062813.112434-3-jaewon02.kim@samsung.com


Signed-off-by: Mark Brown <broonie@kernel.org>