This patch is a CE-optimized assembly implementation for XTS mode.

Benchmark on T-Head Yitian-710 2.75 GHz, the data comes from the 218 mode of
tcrypt, and compared the performance before and after this patch (the driver
used before this patch is xts(ecb-sm4-ce)). The abscissas are blocks of
different lengths. The data is tabulated and the unit is Mb/s:

Before:

xts(ecb-sm4-ce) |      16       64      128      256     1024     1420     4096
----------------+--------------------------------------------------------------
        XTS enc |  117.17   430.56   732.92  1134.98  2007.03  2136.23  2347.20
        XTS dec |  116.89   429.02   733.40  1132.96  2006.13  2130.50  2347.92

After:

xts-sm4-ce      |      16       64      128      256     1024     1420     4096
----------------+--------------------------------------------------------------
        XTS enc |  224.68   798.91  1248.08  1714.60  2413.73  2467.84  2612.62
        XTS dec |  229.85   791.34  1237.79  1720.00  2413.30  2473.84  2611.95
Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>