Makefile: rockchip: Suffix platform type with tpl name

Most of the platforms uses the platform type on their boot
stage image naming conventions in makefile like,

u-boot-x86-start16-tpl.bin - x86 start16 TPL bin
u-boot-spl-mtk.bin - Mediatek SPL bin

This would help to understand the users to what that
particular image belongs to? and less confused.

On that note, suffix platform type rockchip for existing
u-boot-tpl.img so now it become u-boot-tpl-rockchip.bin

Also, bin is more conventional way to include it on tools
like binman, pad_cat etc in future patches.

Note: usage of platform type doesn't follow consistent order
as of now.

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