ADIv5 transport support moves to separate files

ADIv5 transport support moves to separate files

Unclutter arm_adi_v5.c by moving most transport-specific code
to a transport-specific files adi_v5_{jtag,swd}.c ... it's not
a full cleanup, because of some issues which need to be addressed
as part of SWD support (along with implementing the DAP operations
on top of SWD transport):

 - The mess where mem_ap_read_buf_u32() is currently coded to
   know about JTAG scan chains, and thus needs rewriting before
   it will work with SWD;

 - Initialization is still JTAG-specific

Also  move JTAG_{DP,ACK}_* constants from adi_v5.h to the JTAG
file; no other code should care about those values.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>