plat/arm: Move board_css_common.c to juno folder

This file is only used by Juno as all other CSS platforms have their own
private memory maps.

Change-Id: I1c9f27aac7b1d8bff4d92674e8bde5505b93c8c4
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>