fix(rockchip): align fdt buffer on 8 bytes

Since commit 94b2f94b ("feat(libfdt): upgrade libfdt source files"),
8-byte alignment of the FDT address is enforced to follow the DT
standard.

Rockchip implementation of params_early_setup loads the FDT address as
passed by the bootloader into a buffer. This buffer is currently made of
uint8_t which means it is not 8-byte aligned and might result in
fdt_open_into failing.

Instead, let's make this buffer uint64_t to make it 8-byte aligned.

Cc: Quentin Schulz <foss+tf-a@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Change-Id: Ifcf0e0cf4000e3661d76d3c3a2fe3921f7fe44b9