OMAP clock: add omap_clk_get_by_name() for use by OMAP hwmod core code

OMAP clock: add omap_clk_get_by_name() for use by OMAP hwmod core code

The OMAP hwmod core code is intended to use SoC IP block description
structures that are autogenerated from TI's OMAP hardware database.
Currently the hwmod code uses clkdev device + connection addressing to
identify clocks.  This causes problems in the hwmod autogeneration
process, since the TI hardware database doesn't use platform_device or
clkdev addressing; it uses a single clock signal name string, which
tends to bear some resemblance to what is used in the OMAP TRMs.  This
patch adds a non-exported function to the OMAP clock code,
omap_clk_get_by_name().  A subsequent patch will convert the hwmod
code to use this function.

This function is for use only by core code, and practically, no other
code outside the hwmod code should need it.  Device driver code in the
kernel must not use this function, which is why it is not exported.
Drivers should use the appropriate clock alias provided by the clkdev
data structures, so driver code can be completely SoC-independent.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Benoît Cousson <b-cousson@ti.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>