net: pfe_eth: Use spi_flash_read API to access flash memory

Current PFE firmware access spi-nor memory directly. New spi-mem
framework does not support direct memory access. So, let's use
spi_flash_read API to access memory instead of directly using it.

Signed-off-by: Kuldeep Singh <kuldeep.singh@nxp.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>