cmd: bootefi: Honor the address & size cells properties correctly

fdtdec_get_addr_size reads the uses a fixed value for address & size
cell properties which may not be correct always.

Use the auto variant of the function which automatically reads
 #address-cells & #size-cells from parent and uses to read the "reg"
property.

Signed-off-by: Atish Patra <atish.patra@wdc.com>

On 32 bit systems fdt_addr_t may be 4 bytes long but size is defined as
u64. Avoid filling the upper 4 bytes of the sizep parameter of
fdtdec_get_addr_size_auto_parent() with random bytes from the stack by
defining a separate variable fdt_size.

Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>