AArch32: Add generic changes in BL2 ...

This patch adds generic changes in BL2 to support AArch32 state.
New AArch32 specific assembly/C files are introduced and
some files are moved to AArch32/64 specific folders.
BL2 for AArch64 is refactored but functionally identical.

BL2 executes in Secure SVC mode in AArch32 state.

Change-Id: Ifaacbc2a91f8640876385b953adb24744d9dbde3