Platform: Add crypto_keys.c to platform_s

For Musca S1 and B1. In order to allow for a future change to remove the
link between platform_s and platform_bl2 which currently provides the
crypto key symbols to the secure binary.

Change-Id: I9924560af90a734409c1026ed8d40a8af6c26175
Signed-off-by: Raef Coles <raef.coles@arm.com>