mtd: spi-nor: move SECT_4K_PMC special handling

The SECT_4K_PMC flag will set a device specific opcode for the 4k sector
erase. Instead of handling it in the core, we can move it to a
late_init(). In that late init, loop over all erase types, look for the
4k size and replace the opcode.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20220418112650.2791459-1-michael@walle.cc