timer: altera: Remove the codes to get clock frequency

Since we have timer uclass to get clock frequency for us, remove
the custom version in the altera timer driver.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Thomas Chou <thomas@wytron.com.tw>
Acked-by: Simon Glass <sjg@chromium.org>