Handling of AP (and AP register bank) selection, and cached AP
registers, is pretty loose ... start tightening it:

 - It's "AP bank" select support ... there are no DP banks.  Rename.
   + dap_dp_bankselect() becomes dap_ap_bankselect()
   + "dp_select_value" struct field becomes "ap_bank_value"

 - Remove duplicate AP cache init paths ... only use dap_ap_select(),
 and don't make Cortex (A8 or M3) cores roll their own code.

 - For dap_ap_bankselect(), pass up any fault code from writing
 the SELECT register.  (Nothing yet checks those codes.)

 - Add various bits of Doxygen

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