mtd: devices: m25p80: add support for mmap read request

Certain SPI controllers may provide accelerated hardware interface to
read from m25p80 type flash devices in order to provide better read
performance. SPI core supports such devices with spi_flash_read() API.
Call spi_flash_read(), if supported, to make use of such interface.

Signed-off-by: Vignesh R <vigneshr@ti.com>
[Brian: add memset()]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>