target/cortex_a: remove duplicate code to read target registers

target/cortex_a: remove duplicate code to read target registers

The functions cortex_a_dap_{read,write}_coreregister_u32() are
duplicate of the functions dpm_{read,write}_reg().

Remove both duplicated functions in cortex_a.c while export only
dpm_read_reg(), since dpm_write_reg() is currently not used.
Rename dpm_read_reg() as arm_dpm_read_reg() to keep uniform the
naming style.

Change-Id: I501bc99dc402039e630c47917a086a0bb382782c
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4747
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>
Tested-by: jenkins