[ARM] 5390/1: AT91: Watchdog fixes

The recently merged AT91SAM9 watchdog driver uses the
AT91SAM9X_WATCHDOG config variable, whereas the original version of
the driver (and the platform support code) used AT91SAM9_WATCHDOG.
This causes the watchdog platform_device to never be registered, and
therefore the driver not to be initialized.

This patch:
- updates the platform support code to use AT91SAM9X_WATCHDOG.
- includes <linux/io.h> to fix compile error (same fix as was applied
to at91rm9200_wdt.c)
- fixes comment regarding watchdog clock-rates in at91rm9200.

Signed-off-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>