mtd: spi-nor: micron-st: Enable locking for mt25qu256a

mt25qu256a [1] uses the 4 bit Block Protection scheme and supports
Top/Bottom protection via the BP and TB bits of the Status Register.
BP3 is located in bit 6 of the Status Register.
Tested on MT25QU256ABA8ESF-0SIT.

[1] https://www.micron.com/-/media/client/global/documents/products/data-sheet/nor-flash/serial-nor/mt25q/die-rev-a/mt25q_qljs_u_256_aba_0.pdf

Signed-off-by: Eliav Farber <farbere@amazon.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20221020092058.33844-1-farbere@amazon.com