sched: walt: Move initialization to walt.c

All of the WALT specific initialization in sched_init
can be done in walt.c. Add a function walt_sched_init
to walt.c and move all the initialization there. This
will allow for easier updates to the init code in
upcoming patches.

Change-Id: I2344c2cf02d84b876650350581f88a58041f4ccc
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>