fvp: Introduce mixed compilation mode 32bits / 64bits

This patch introduce compilation mode 32bits / 64 bits independently
for every parts:
- COMPILE_NS_USER:   Non-Secure User
- COMPILE_NS_KERNEL: Non-Secure Kernel
- COMPILE_S_USER:    Secure User
- COMPILE_S_KERNEL:  Secure Kernel

This is used on FVP, the non-secure part being bits, but the secure
part can be specified as 32 or 64 bits (64bits by default).

Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Pascal Brand <pascal.brand@linaro.org> (QEMU)
Tested-by: Pascal Brand <pascal.brand@linaro.org> (FVP)
Signed-off-by: Pascal Brand <pascal.brand@st.com>