ARM926: more cleanup

An init_target() wrapper isn't needed, and target_create()
can shrink a bit.  Add a header comment and some doxygen.

Remove arm926ejs_catch_broken_irscan() which has been a NOP
for quite a few months now, and in any case duplicates logic
in the JTAG core to validate IR capture data.  But force the
capture mask to 0x0f, so those tests are most effective.

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