scripts: extract_dts_includes: Generate'_0' defines only when needed

Indexed defines were systematically generated even when there
was only one element to generate.
So we ended up generated a lot of _0 defines.
Then we needed to generate aliases to these _0 indexed defines,
in order to get useful defines.
For instance:
 #define GPIO_LEDS_0_GPIO_FLAGS_0	4
 #define GPIO_LEDS_0_GPIO_PIN_0		5
 #define GPIO_LEDS_0_GPIO_FLAGS		GPIO_LEDS_0_GPIO_FLAGS_0
 #define GPIO_LEDS_0_GPIO_PIN		GPIO_LEDS_0_GPIO_PIN_0

This commit allows to generate _0 indexed define only if a
property has more than one elements to define.
Aliases generation to _0 indexed defines are also removed.

Note: IRQ are left untouched since this is frequent to handle
multiple IRQs in a driver

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>