Kconfig: Add default SPL_FIT_GENERATOR for rockchip

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 <[email protected]>
Signed-off-by: Jagan Teki <[email protected]>
Reviewed-by: Kever Yang <[email protected]>