armv8/fsl-lsch3: fdt: Check the pointer returned from call to a function may be NULL

Pointer 'reg' returned from call to function 'fdt_getprop' may be
NULL, will be passed to function and may be dereferenced there by
passing argument 1 to function 'of_read_number'. So check pointer
'reg' first.

Signed-off-by: Alison Wang <alison.wang@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>