MPC824x: remove obsolete "PN62" board

The MPC824x processors have long reached EOL, and the PN62 board has
not seen any board-specific updates for more than a decade.  It is now
causing build issues.  Instead of wasting time on things nobody is
interested in any more, we rather drop this board.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Wolfgang Grandegger <wg@grandegger.com>
cc: Tom Rini <trini@ti.com>