remoteproc: fix argument 2 of rproc_mem_entry_init

The rproc_mem_entry_init() call takes a pointer to a vm
as the second argument. The code is currently using a
plain 0 as "NULL". Change to using NULL to fix the
following sparse warnings:

drivers/remoteproc/remoteproc_core.c:339:49: warning: Using plain integer as NULL pointer
drivers/remoteproc/remoteproc_core.c:916:46: warning: Using plain integer as NULL pointer

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>