FVP: Add TB_FW_CONFIG and HW_CONFIG

This patch adds TB_FW_CONFIG for FVP and allows FVP
to select the appropriate HW_CONFIG to include in the
fip. The HW_CONFIG for FVP is selected via `FVP_HW_CONFIG_DTS`
build option. The TB_FW_CONFIG specifies the load address of
HW_CONFIG to BL2. Since currently the load address is different
between AARCH32 and AARCH64, 2 separate TB_FW_CONFIGs are
maintained for the 2 modes.

Change-Id: Ide8581e752dfa900087f5895c775073c841c0daf
Signed-Off-By: Soby Mathew <soby.mathew@arm.com>