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>