Kconfig: Add default SPL_FIT_GENERATOR for rockchip

Add default SPL_FIT_GENERATOR py script for rockchip platforms if
specific target enabled SPL_LOAD_FIT.

So, this would help get rid of explicitly mentioning the default
SPL FIT generator in defconfigs. however some targets, like puma_rk3399
still require their own FIT generator so in those cases the default will
override with defconfig defined generator.

Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>