remoteproc: move rproc_da_to_va declaration to remoteproc.h

The rproc_da_to_va() API is an exported function, so move its
declaration from the remoteproc local remoteproc_internal.h
to the public remoteproc.h file.

This will allow drivers outside of the remoteproc folder to be
able to use this API. Without this, a build issue is seen when
this API is used from the wkup_m3_ipc driver on TI AM335x/AM437x
SoCs.

Signed-off-by: Suman Anna <s-anna@ti.com>