remoteproc: k3-dsp: Refactor mbox request code in start

Refactor out the mailbox request and associated ping logic code
from k3_dsp_rproc_start() function into its own separate function
so that it can be re-used in the soon to be added .attach() ops
callback.

Signed-off-by: Suman Anna <s-anna@ti.com>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220213201246.25952-5-s-anna@ti.com