Drivers gic and uart are optional

Platforms stm do not use gic and uart drivers, whereas vexpress (fvp / qemu)
does. So the conf.mk of the latter case defines the following:
    WITH_UART_DRV := y
    WITH_GIC_DRV := y

Reviewed-by: Joakim Bech joakim.bech@linaro.org
Reviewed-by: Jens Wiklander jens.wiklander@linaro.org
Signed-off-by: Pascal Brand <pascal.brand@st.com>