ENGR00303418 mtd: fsl-quadspi: set AHB transfer size to the maximum value

ENGR00303418 mtd: fsl-quadspi: set AHB transfer size to the maximum value

Set AHB transfer size to 1K which significantly improved the read
performance.

Add ahb_buf_size field in fsl_qspi_devtype_data to denote the size for
different SoC.

Before:

root@imx6qdlsolo:~# dd if=/dev/mtd1 of=/dev/null bs=1M count=16
16+0 records in
16+0 records out
16777216 bytes (17 MB) copied, 0.472183 s, 35.5 MB/s

After:

root@imx6qdlsolo:~# dd if=/dev/mtd1 of=/dev/null bs=1M count=16
16+0 records in
16+0 records out
16777216 bytes (17 MB) copied, 0.369439 s, 45.4 MB/s

Signed-off-by: Allen Xu <b45815@freescale.com>
Signed-off-by: Huang Shijie <b32955@freescale.com>