[WATCHDOG] ar7_wdt: fix path to ar7-specific headers

AR7 is currently being resubmitted for mainline inclusion
and we changed the path to the ar7-specific headers
from ar7 to mach-ar7 to reflect the other MIPS-based
boards header hierarchy. This patch will avoid any future
compilation failure due to missing headers.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>