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In order to access MG SRAM, the amb bridge needs to be configured which is
done in bl2 platform init.

For MG CM3, the image is only loaded to its SRAM and the CM3 itself is
left in reset. It is because the next stage bootloader (e.g. u-boot)
will trigger action which will take it out of reset when needed. This
can happen e.g. when appropriate device-tree setup (which has enabled
802.3 auto-neg) will be chosen. In other cases the MG CM3 should not be
running.

Change-Id: I816ea14e3a7174eace068ec44e3cc09998d0337e
Signed-off-by: Grzegorz Jaszczyk <jaz@semihalf.com>