xen: re-define assign_pages and introduce a new function assign_page

xen: re-define assign_pages and introduce a new function assign_page

In order to deal with the trouble of count-to-order conversion when page number
is not in a power-of-two, this commit re-define assign_pages for nr pages and
assign_page for original page with a single order.

Backporting confusion could be helped by altering the order of assign_pages
parameters, such that the compiler would point out that adjustments at call
sites are needed.

[stefano: switch to unsigned int for nr]
Signed-off-by: Penny Zheng <penny.zheng@arm.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>