ARM: OMAP2+: PRM: provide generic API for system reset

This patch combines the various prm_warm_reset calls under a common
API prm_reset_system, and adds the SoC specific implementation under
prm_ll_data.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Acked-by: Paul Walmsley <paul@pwsan.com>
Tested-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>