dtb_platdata.py: prevent define generation of alias

dtb_platdata.py: prevent define generation of alias

generate define for an alias only if the struct is not
created already.

This prevents compilerwarning:
  PLAT    spl/dts/dt-platdata.o
spl/dts/dt-platdata.c:11:46: error: missing braces around initializer [-Werror=missing-braces]
 static const struct dtd_simple_bus dtv_ahb = {
                                              ^
spl/dts/dt-platdata.c:20:46: error: missing braces around initializer [-Werror=missing-braces]
 static const struct dtd_simple_bus dtv_apb = {
                                              ^
cc1: all warnings being treated as errors

on the at91 based taurus board. Reason is in at91sam9260.dtsi
is defined:

ahb {
        compatible = "simple-bus";
        ranges;

and later:

pinctrl: pinctrl@fffff400 {
        compatible = "atmel,at91rm9200-pinctrl", "simple-bus";
        ranges = <0xfffff400 0xfffff400 0x600>;

without this patch dtoc generates:

struct dtd_atmel_at91rm9200_pinctrl {
	fdt32_t		atmel_mux_mask[6];
	fdt32_t		ranges[3];
	fdt32_t		reg[6];
};

struct dtd_simple_bus {
        bool            ranges;
};

"#define dtd_simple_bus dtd_atmel_at91rm9200_pinctrl"

and the line with "define dtd_simple_bus..." introduces
the warning. This define is not needed.

Signed-off-by: Heiko Schocher <hs@denx.de>