driver/mtd/spi:Read 8KB data chunk during u-boot load in SPL

driver/mtd/spi:Read 8KB data chunk during u-boot load in SPL

SPI driver perform its operation(read/write) on 64KB buffer chunk for data
greater than 64KB. This buffer chunk is allocated from system heap.

During SPL boot, 768KB of data is read from SPI flash.
Here, heap size may not be sufficient enough to full-fill 64KB buffer
requirement of SPI driver. So break down u-boot read operation at 8KB of chunk.

Also, fix a warning i.e. "unused variable buf" during CONFIG_FSL_CORENET

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>