clocksource: dw_apb_timer_of: Fix support for dts binding "snps,dw-apb-timer"

In commit 620f5e1cbf (dts: Rename DW APB timer compatible strings), both
"snps,dw-apb-timer-sp" and "snps,dw-apb-timer-osc" were deprecated in place
of "snps,dw-apb-timer". But the driver also needs to be udpated in order to
support this new binding "snps,dw-apb-timer".

Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>