ARM: imx: deconstruct mx3_idle

The imx31 and imx35 idle functions are almost the same, but we
currently have to check the cpu type every time. This can be
simplified by moving the logic from mx3_cpu_lp_set() into
two separate idle functions, removing the last user of
cpu_is_mx35.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>