hla: API: specify that read_reg/write_reg use regsel as parameter

hla: API: specify that read_reg/write_reg use regsel as parameter

The API of hla have been defined from ST-Link basic operations.
By chance, all the current implementation of hla (st-link, ti-icdi
and nulink) share the same way to handle the parameter 'num' of
the API read_reg() and write_reg(), that is simply using it to
initialize the field REGSEL (bits [6:0]) of armv7m Debug Core
Register Selector Register (DCRSR).

Add a comment in the API definition to highlight this, in case it
get used by a further hla implementation, then rename as 'regsel'
the 'num' parameter.

Change-Id: I4b6a2c7d78b4fc4de2b9b2bdba065414b15c6ba3
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5882
Reviewed-by: Edward Fewell <edwardfewell@hotmail.com>
Tested-by: jenkins
Reviewed-by: Zale Yu
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>