mtd: spi-nor: move all micron-st specifics into micron-st.c

The flag status register is only available on micron flashes. Move all
the functions around that into the micron module.

This is almost a mechanical move except for the spi_nor_fsr_ready()
which now also checks the normal status register. Previously, this was
done in spi_nor_ready().

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Tested-by: Pratyush Yadav <p.yadav@ti.com> # on mt35xu512aba, s28hs512t
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20220223134358.1914798-25-michael@walle.cc