riscv: Move early fdt mapping creation in its own function

The code that handles the early fdt mapping is hard to read and does not
create the same mapping size depending on the kernel:

- for 64-bit, 2 PMD entries are used which amounts to a 4MB mapping
- for 32-bit, 2 PGDIR entries are used which amounts to a 8MB mapping

So keep using 2 PMD entries for 64-bit and use only one PGD entry for
32-bit needed to cover 4MB. Move that into a new function called
create_fdt_early_page_table which, using the same naming as
create_kernel_page_table.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>