spl: make image arg or fdt blob address reconfigurable

At present fdt blob or argument address being passed to kernel is fixed at
compile time using macro CONFIG_SYS_SPL_ARGS_ADDR. FDT blob from
different media like nand, nor flash are copied to the address pointed
by the macro.
The problem is, it makes args/fdt blob compulsory to copy which is not required
in cases like for NOR Flash. This patch removes this limitation.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>