spi: hisi-kunpeng: Add debugfs support

This patch uses debugfs_regset32 interface to create the registers dump
file. Use it instead of creating a generic debugfs file with manually
written read callback function.

With these entries, users can check all the SPI controller registers
during run time.

Signed-off-by: Jay Fang <f.fangjian@huawei.com>
Link: https://lore.kernel.org/r/1622789718-13977-1-git-send-email-f.fangjian@huawei.com
Signed-off-by: Mark Brown <broonie@kernel.org>