OMAP3 clock: reorganize CK_* platform flags

Add CK_* flags for the two new Sitara chips, AM3505 and AM3517, and
the OMAP34xx die shrink, OMAP36xx/OMAP37xx.  Introduce a new CK_*
flag, CK_3XXX, that marks all clocks that are common to OMAP3 family
chips.  CK_343X now refers to clocks that are available only on
OMAP34{1,2,3,4}0 (WTBU) and OMAP35{03,15,25,30} (any version).
At some point, the RATE_IN_* flags should be updated also.

While here, add some documentation describing the chip families
covered by these clock flags.

This patch is partially based on patches from Ranjith Lohithakshan
<ranjithl@ti.com> and Vishwanath Sripathy <vishwanath.bs@ti.com>.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Ranjith Lohithakshan <ranjithl@ti.com>
Cc: Vishwanath Sripathy <vishwanath.bs@ti.com>