scsi: ufs: ufs-exynos: Support custom version of ufs_hba_variant_ops

By default, ufs_hba_exynos_ops will be used. Add support for a custom
version of ufs_hba_variant_ops because some variants of exynos-ufs will use
only few callbacks.

Link: https://lore.kernel.org/r/20211018124216.153072-8-chanho61.park@samsung.com
Cc: Alim Akhtar <alim.akhtar@samsung.com>
Cc: Kiwoong Kim <kwmad.kim@samsung.com>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>