Add HWMOD_EXT_OPT_MAIN_CLK flag to indicate that this IP block is
dependent on an off-chip functional clock that is not guaranteed to be
present during initialization.  IP blocks marked with this flag are
left in the INITIALIZED state during kernel init.

This is a workaround for a hardware problem.  It should be possible to
guarantee that at least one clock source will be present and active
for any IP block's main functional clock.  This ensures that the hwmod
code can enable and reset the IP block.  Resetting the IP block during
kernel init prevents any bogus bootloader, ROM code, or previous OS
configuration from affecting the kernel.  Hopefully a clock
multiplexer can be added on future SoCs.

N.B., at some point in the future, it should be possible to query the
clock framework for this type of information.  Then this flag should
no longer be needed.
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Benoît Cousson <b-cousson@ti.com>