remoteproc: Use struct_size() helper

One of the more common cases of allocation size calculations is finding
the size of a structure that has a zero-sized array at the end, along
with memory for some number of elements for that array. For example:

struct resource_table {
	...
        u32 offset[0];
} __packed;

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

So, replace the following form:

table->num * sizeof(table->offset[0]) + sizeof(struct resource_table)

with:

struct_size(table, offset, table->num)

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>