misc: fastrpc: Add fastrpc_remote_heap_alloc

misc: fastrpc: Add fastrpc_remote_heap_alloc

Split fastrpc_buf_alloc in such a way it allows allocation of remote
heap too and add fastrpc_remote_heap_alloc to do so.

Co-developed-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20221125071405.148786-5-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>