clocksource: update "fn" at CLOCKSOURCE_OF_DECLARE() of nps400 timer

nps_setup_clocksource() should take node as only argument as defined by
typedef int (*of_init_fn_1_ret)(struct device_node *)

Therefore need to replace:
int __init nps_setup_clocksource(struct device_node *node, struct clk *clk)
with
int __init nps_setup_clocksource(struct device_node *node)

This patch also serve as preparation for next patch which add support
for clockevents to nps400.
Specifically we add new function nps_get_timer_clk() to serve clocksource
and later clockevent registration.

Signed-off-by: Noam Camus <noamca@mellanox.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>