char: virtio_fastrpc: Add non-contiguous buffer alloction support

SNPE applications need to allocate hundreds of MB buffer. But
there is only 32MB CMA in GVM. So we convert to use alloc_pages
in fastrpc_buf_alloc. And these buffers don't need to be contiguous.

Change-Id: I07c8836548fa1143fe53df9f6094ad2f6c409626
Signed-off-by: Ping Jiang <scottjiang@codeaurora.org>