ARM: PNX4008: convert watchdog to use clk API enable/disable calls

clk_set_rate() is not supposed to be used to turn clocks on and off.
That's what clk_enable/clk_disable is for.

Acked-by: Wim Van Sebroeck <wim@iguana.be>
Acked-by: Vitaly Wool <vitalywool@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>