board_f: Drop the timer after relocation

Once U-Boot relocates itself the existing driver-model timer (if any) is
no-longer valid until the device is reinitialised. Any use of the device
may cause a crash. To handle this, set the timer to NULL after relocation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>