clk: si5351: Rename internal plls to avoid name collisions

This drivers probe fails due to a clock name collision if a clock named
'plla' or 'pllb' is already registered when registering this drivers
internal plls.

Fix it by renaming internal plls to avoid name collisions.

Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Cc: Rabeeh Khoury <rabeeh@solid-run.com>
Signed-off-by: Sergej Sawazki <sergej@taudac.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>