flash/nor/atsame5: Fix a timeout when erasing

According to the datasheet, erasing a block can take up to 200 ms.
When using a Segger J-Link with a 2 MHz clock the current loop
finishes after < 50 ms, ignores the timeout, and then fails when
erasing the next block.

Switch to a time based check, add an explicit yield, and report an
error on timeout.

Change-Id: I8255401d1e59f427a08d2cccb8a66143dcdbb324
Signed-off-by: Michael Hope <mlhx@google.com>
Reviewed-on: http://openocd.zylin.com/5706
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>