spi: mt7621-spi: remove data cache and rewrite its xfer function

spi: mt7621-spi: remove data cache and rewrite its xfer function

The mt7621 spi controller supports continuous generic half-duplex spi
transaction. There is no need to cache xfer data at all.

To achieve this goal, the OPADDR register must be used as the first data
to be sent. And follows the eight generic DIDO registers. But one thing
different between OPADDR and DIDO registers is OPADDR has a reversed byte
order.

With this patch, any amount of data can be read/written in a single xfer
function call.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>