ARM: OMAP2+: hwmod: fetch main_clk based on hwmod name

With the transition to hwmod module clocks, all hwmods will have
their main clocks named <hwmod_name>_mod_ck. Use this info to
fetch main_clk, and use it if found.

Also, if a main_clk is found based on the hwmod name, disable
the direct PRCM modulemode access from hwmod.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>