teec: use multiple of page size for page aligned buffers

When allocating a page aligned buffer, round the size up the next
multiple of page size. With this we can guarantee that a part of that
page isn't registered already.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>