config: Enable AVTIMER_LEGACY for sdm450

For targets using avtimer from camera
kernel driver, add config AVTIMER_LEGACY to
initialize avtimer function pointers to use by camera
driver to get avtimer tick.

CRs-Fixed: 2201340
Change-Id: Ibda01617a77ff2cc172572e213a7ecca6ab97d43
Signed-off-by: Laxminath Kasam <lkasam@codeaurora.org>