clocksource/drivers/dw_apb_timer_of: Add error handling if no clock available

commit  4d47e539cd9894d64e6f3aa1dca7cedce4486d77 from
https://github.com/altera-opensource/linux-socfpga.git

[upstream commit 5d9814df]

commit ("b0fc70ce

 arm64: berlin: Select DW_APB_TIMER_OF") added the
support for the dw_apb_timer into the arm64 defconfig. However, for some
platforms like the Intel Stratix10 and Agilex, the clock manager doesn't
get loaded until after the timer driver get loaded. Thus, the driver hits
the panic "No clock nor clock-frequency property for" because it cannot
properly get the clock.

This patch adds the error handling needed for the timer driver so that
the kernel can continue booting instead of just hitting the panic.

Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201205105223.208604-1-dinguyen@kernel.org
Signed-off-by: Meng Li <Meng.Li@windriver.com>