clk: ingenic: Handle setting the Low-Power Mode bit

The Low-Power Mode, when enabled, will make the "wait" MIPS instruction
suspend the system.

This is not really clock-related, but this bit happens to be in the
register set of the CGU.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>