MIPS: jz4740: PM: Let CGU driver suspend clocks and set sleep mode

Instead of forcing the jz4740 clocks to suspend here, we let the CGU
driver handle it.
We also let the CGU driver set the "sleep mode" bit.

This has the added benefit that now it is possible to build a kernel on
SoCs newer than the JZ4740 with CONFIG_PM.

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