mtd: fsl-quadspi: improve read performance by increase AHB transfer size

Set AHB transfer size to 1K which 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, 25.1 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, 29.5 MB/s

Signed-off-by: Allen Xu <b45815@freescale.com>
Signed-off-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Frank Li <Frank.Li@freescale.com>
Acked-by: Huang Shijie <shijie.huang@intel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>