ARM: dts: imx53-smd: Remove invalid SPI flash entry

"st,m25p" is not a valid compatible according to jedec,spi-nor.yaml.

Remove it.

Also, node names should be generic, so use 'flash' as the SPI NOR flash
node name.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>