Tegra: clk: always use find_best_divider() for periph clocks

When adjusting peripheral clocks always use find_best_divider()
instead of clk_get_divider() even when a secondary divider is not
available.  In the case where is requested clock is too slow to be
derived from the parent clock this allows a best effort to get close
to the requested clock.

This comes up for commands like "sf" where the user can pass a clock
speed on the command line or "sspi" where the clock is hardcoded to
1MHz, but the Tegra114 SPI controller can't go that low.

Signed-off-by: Allen Martin <amartin@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>