spi: Drop obsolete mtk_qspi driver references

Since u-boot has added the spi-mem framework and replaced
the spi-nor framework, the mtk_qspi is no longer compatible
with the new spi-nor driver.

Remove this driver along with replacing config item
with new mtk spi-nor driver.

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>
[jagan: squash related changes and update commit message]
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>