remoteproc: add alloc ops in rproc_mem_entry struct

Memory entry could be allocated in different ways (ioremap,
dma_alloc_coherent, internal RAM allocator...).
This patch introduces an alloc ops in rproc_mem_entry structure
to associate dedicated allocation mechanism to each memory entry
descriptor in order to do remote core agnostic from memory allocators.

The introduction of this ops allows to perform allocation of all registered
carveout at the same time, just before calling rproc_start().
It simplifies and makes uniform carveout management whatever origin.

Signed-off-by: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>