arch: remove BR2_arm920 reference

The BR2_GCC_TARGET_CPU defines a value for the BR2_arm920 case, but
this option does not exist. Therefore, this commit removes one line of
dead code.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>