input: sensors: support continuous mode read for akm8963

akm8963 compass operating mode can be single measure or
continuous. Use single measure mode for low frequency operations
and use continuous measure mode for high frequency operations.

Change-Id: I1aea7dc946ea205dea7194d2ce2e8a15df71c560
Signed-off-by: Ananda Kishore <kananda@codeaurora.org>