ARM: OMAP2+: hwmod: use init-time function pointer for wait_target_ready

Rather than using cpu_is* checking at runtime, initialize an SoC specific
function pointer for wait_target_ready().

While here, downgrade the BUG() to a WARN_ON() so it gives a noisy
warning instead of causing a kernel panic.

Signed-off-by: Kevin Hilman <khilman@ti.com>
[paul@pwsan.com: convert to use soc_ops function pointers; add kerneldoc;
 move soc_ops functions to their own section in the code; integrated
 the _wait_target_ready() function with the OMAP2/OMAP4 variants;
 renamed the wait_module_ready field to wait_target_ready]
Signed-off-by: Paul Walmsley <paul@pwsan.com>