boot: fdt: Turn all addresses and sizes into u64

In case of systems where DRAM bank ends at the edge of 32bit boundary,
start + size calculations would overflow. This happens on STM32MP15xx
with 1 DRAM bank starting at 0xc0000000 and 1 GiB of DRAM. This is a
usual 32bit system DRAM size overflow, fix it by doing all DRAM size
and offset calculations using u64 types. This also covers a case where
a 32bit PAE system might be able to address up to 36bits of DRAM.

Fixes: a4df06e4 ("boot: fdt: Change type of env_get_bootm_low() to phys_addr_t")
Signed-off-by: Marek Vasut <marex@denx.de>