sched: remove "hmp" references in the code

HMP specific code is removed in the previous patches. There are
still references to HMP in the code that is shared with WALT.
Rename hmp_stats and hmp_flags methods in rq structure to
walt_stats and walt_flags.

Change-Id: I37f7ef1893e479de0e8a2c3b809a7e2e631adead
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>