QSPI may failed to map enough memory (256MB) for AHB read in
previous implementation, especially in 3G/1G memory layout kernel.
Dynamically map memory to avoid such issue.

This implementation generally map QUADSPI_MAX_IOMAP (default 4MB) memory
for AHB read, it should be enough for common scenarios, and the side
effect (0.6% performance drop) is minor.

Previous implementation

root@imx6qdlsolo:~# dd if=/dev/mtd0 of=/dev/null bs=1K count=32K
32768+0 records in
32768+0 records out
33554432 bytes (34 MB) copied, 2.16006 s, 15.5 MB/s

root@imx6qdlsolo:~# dd if=/dev/mtd0 of=/dev/null bs=32M count=1
1+0 records in
1+0 records out
33554432 bytes (34 MB) copied, 1.43149 s, 23.4 MB/s

After applied the patch

root@imx6qdlsolo:~# dd if=/dev/mtd0 of=/dev/null bs=1K count=32K
32768+0 records in
32768+0 records out
33554432 bytes (34 MB) copied, 2.1743 s, 15.4 MB/s

root@imx6qdlsolo:~# dd if=/dev/mtd0 of=/dev/null bs=32M count=1
1+0 records in
1+0 records out
33554432 bytes (34 MB) copied, 1.43158 s, 23.4 MB/s
Signed-off-by: Han Xu <han.xu@freescale.com>
Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>