arm: dts: dra7-evm: add spi-flash comaptible for flash

In U-Boot most flashes uses "spi-flash" as compatible to bind the
device to flash driver, so adding "spi-flash" compatible to
m25p80 node.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>