[WATCHDOG] fix book E watchdog to take WDIOC_SETTIMEOUT arg in seconds

The WDIOC_SETTIMEOUT argument is supposed to be a "seconds" value.
However, the book E wdt currently treats it as a "period" which is
interpreted in a board-specific way.

This patch allows the user to pass in a "seconds" value and the driver
will set the smallest timeout that is at least as large as specified
by the user.  It's been tested on e500 hardware and works as
expected.

The patch only modifies the CONFIG_FSL_BOOKE case, the CONFIG_4xx case
is left unmodified as I don't have any hardware to test it on.

Signed-off-by: Chris Friesen <cfriesen@nortel.com>
Cc: Kumar Gala <galak@gate.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>