ARM: OMAP3+: dpll: Configure autoidle mode only if it's supported

The current DPLL code enables and disables autoidle features
without checking whether the autoidle register is available.
Fix this by putting a check for the existence of the autoidle
register in the DPLL data.

With such a check in place, for DPLLs which do not support this
feature, simply skipping the autoidle_reg entry in the DPLL data
is sufficient.

Signed-off-by: Vaibhav Bedia <vaibhav.bedia@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>