Reading and writing with different bit width need to be supported for SPI rx and tx registers

[YdrMaster]

Document of SPI_RXD:

This register can be accessed in the byte, half-word, or word unit by AHB. In the byte accessing method, if there are data in RXFIFO, the top word is returned and the RXFIFO depth is decreased by 1. In the half-word accessing method, two SPI bursts are returned and the RXFIFO depth is decreased by 2. In the word accessing method, the four SPI bursts are returned and the RXFIFO depth is decreased by 4.

These registers respond to different load (lb, lh and lw) and store (sb, sh and sw) instructions in different ways.