arm: common: Use label to identify cpaccess dummy instruction

Use a label instead of using function start address. It's not
guaranteed that the compiler will make the mrc instruction
the first instruction in the function.

Self-modifying code replaces the dummy instruction with a new
instruction during runtime according to user inputs.

Change-Id: I8c37f4e414ce5983f0317a51a3cf790b45763bfc
Acked-by: Suren Eda Naarayana Kulothungan <sedanaar@qualcomm.com>
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>