of: Fix memory block alignment in early_init_dt_add_memory_arch()

If a memory block is not aligned to PAGE_SIZE, its base address must be
rounded up, not down, and its size must be reduced.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Grant Likely <grant.likely@linaro.org>