ARMV7: Fix duplicate use of "b" parameter in ACTIM_CTRLA definition

ACTIM_CTRLA macro errently passes "b" parameter to ACTIM_CTRLA_TRAS()
instead of "c".  To make usage more clear, replace all single-letter
macro parameters with more descriptive parameter names.

Signed-off-by: Peter Barada <peter.barada@logicpd.com>