OMAP4: clock: Add optional clock nodes

OMAP4 IP optional clocks require explicit enable in module CTRLCLK
register. In order to allow that we have to create artificial clock
nodes that represent this clock inputs in the IP.

Notes:
- Temporary use OMAP3 names for GPIO optional clocks until the GPIO hwmod
convertion is done. It will enforce the usage of OMAP4 names as the reference.
- Temporary use OMAP3 names for TIMER main clock (gptX_fck) until TIMER hwmod
convertion is done. During that convertion, the new name will have to be used.

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