clk: pistachio: Add a pll_lock() helper for clarity

This commit adds a pll_lock() helper making the code more readable.
Cosmetic change only, no functionality changes.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>