remoteproc: fix rproc_alloc_carveout() bad variable cast

As dma member of struct rproc_mem_entry is dma_addr_t, no
need to cast in u32.

Fixes: d7c51706 ("remoteproc: add alloc ops in rproc_mem_entry struct")

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