target/cortex_a: use extensively cortex_a_wait_dscr_bits()

target/cortex_a: use extensively cortex_a_wait_dscr_bits()

We have the function to wait for bits in dscr. Use it whenever
possible.

When the bit to test is DSCR_INSTR_COMP, use instead the wrapper
function cortex_a_wait_instrcmpl().

Change-Id: I5c54c239a00f489712af448eb97752210b4b38b8
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5113
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>