boot: fdt: Fix tmp type in env_get_bootm_size() and rename to low

Change type of 'tmp' variable from phys_size_t to phys_addr_t and
rename it to 'low' to better describe what the variable represents,
which is either the bootm_low address from environment or start of
DRAM address.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>