Based on the Linux v4.4 patch

commit c67cbb8
Author:     Brian Norris <computersforpeace@gmail.com>
AuthorDate: Tue Nov 10 12:15:27 2015 -0800
Commit:     Brian Norris <computersforpeace@gmail.com>
CommitDate: Thu Nov 19 13:34:44 2015 -0800

    mtd: spi-nor: provide default erase_sector implementation

    Some spi-nor drivers perform sector erase by duplicating their
    write_reg() command. Let's not require that the driver fill this out,
    and provide a default instead.

    Tested on m25p80.c and Medatek's MT8173 SPI NOR flash driver.

    Signed-off-by: Brian Norris <computersforpeace@gmail.com>

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>