psoc6: Run flash algorithm asynchronously to improve performance

psoc6: Run flash algorithm asynchronously to improve performance

Existing psoc6 driver starts flash algorithm for each Flash row. This is
suboptimal from performance point of view, starting/stopping flash
algorithm for each row adds significant overhead. This change starts
flash algorithm and leaves it running asynchronously while driver
performs flash operations.

Performance gain is 170...250% depending on probe:

flash write_image img_256k.bin    | w/o this change | with this change |
----------------------------------|-----------------|------------------|
KitProg2/CMSIS-DAP, SWD @ 1 MHz   |     4 KiB/s     |     10 KiB/s     |
J-Link Ultra, SWD @ 1 MHz         |    17 KiB/s     |     31 KiB/s     |
J-Link Ultra, SWD @ 4 MHz         |    33 KiB/s     |     57 KiB/s     |

Change-Id: I5bd582584b35af67600c4d197829eb7aeeec7e3f
Signed-off-by: Bohdan Tymkiv <bhdt@cypress.com>
Reviewed-on: http://openocd.zylin.com/4472
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>