rk27xx: support entering rkusb mode on rk2706A

We know about two different bootroms. First can be found in rk2706A,
the second in rk2706B and rk2705. This two versions are very
similar but memory addresses are different. It seems it is possible
to distinguish bootrom version by reading SCU_ID register.

Change-Id: I01681b5e3a82930ae74a5cce6ab0244d7cd333d2
1 file changed