rockchip: drop rockchip-specific imported linker symbols for bl31

In the rockchip bl31 setup the __RO_START__ and __RO_END__ symbols are
currently imported into special BL31_RO_* constants while the general
code also imports them as BL_CODE_BASE and BL_CODE_END.

So we can just use the general symbols and can drop the duplication.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Change-Id: Ibf1b48ad80bed897247a1690a32711030479262d