ARM: lpc18xx: define low-level debug symbol for LPC18xx/43xx

Using a dedicated symbol for low-level debugging instead of the
arch symbol will make this platform play nice when enabled on a
kernel that supports multiple platforms.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>