drivers: spi: add et171 spi driver

The Egis ET171 SPI controller was based on Andes ATCSPI200,
but has since been modified. In particular, the WrTranCnt and RdTranCnt
fields from the SPI Transfer Control Register have been moved to dedicated
Wr_Tran_Cnt and Rd_Tran_Cnt registers.

Signed-off-by: jacob kung <jacob.kung@egistec.com>