rpi3: Set RPI3_PRELOADED_DTB_BASE for TF-A

In TF-A in the commit:
   (5341b42ec) rpi3: Add mem reserve region to DTB if present

they started to runtime patching of the DTB, but the default build rules
in TF-A doesn't set RPI3_PRELOADED_DTB_BASE to anything which then gets
set to the default value: 1, which gives incorrect memory mappings for
MAP_BL32_MEM. Therefore we must tell where the DTB is located and we do
that by adding adding a compile time flag to TF-A, i.e.,

       RPI3_PRELOADED_DTB_BASE=0x00010000

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Joakim Bech <joakim.bech@linaro.org> (RPi3)
Reviewed-by: Igor Opaniuk <igor.opaniuk@linaro.org>