boot: fdt: Change type of env_get_bootm_low() to phys_addr_t

Change type of ulong env_get_bootm_low() to phys_addr_t env_get_bootm_low().
The PPC/LS systems already treat env_get_bootm_low() result as phys_addr_t,
while the function itself still returns ulong. This is potentially dangerous
on 64bit systems, where ulong might not be large enough to hold the content
of "bootm_low" environment variable. Fix it by using phys_addr_t, similar to
what env_get_bootm_size() does, which returns phys_size_t .

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reported-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>