imx:mx6sxsabresd support qspi AHB read

Add CONFIG_SYS_FSL_QSPI_AHB in header file to enable AHB in driver.
In order to count the time, add CONFIG_CMD_TIME.

Using AHB read can improve the the read speed about 30%.

AHB read:
=> time sf read 0x8f800000 0 100000
SF: 1048576 bytes @ 0x0 Read: OK
time: 0.174 seconds

=> time sf read 0x8f800000 1000000 100000
SF: 1048576 bytes @ 0x1000000 Read: OK
time: 0.174 seconds

IP read:
=> time sf read 0x8f800000 0 100000
SF: 1048576 bytes @ 0x0 Read: OK
time: 0.227 seconds

=> time sf read 0x8f800000 1000000 100000
SF: 1048576 bytes @ 0x1000000 Read: OK
time: 0.227 seconds

Note:
Quad read is not supported in driver, now. In my side, using AHB and Quad read
can achieve about 16MB/s. Anyway, I have plan to reimplement the driver using
DTB and DM, then make the code cleaner and more feature can be added.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>