Add GENMASK macros

Import GENMASK_32 and GENMASK_64 macros from optee-os (permissive license).
And default GENMASK is set to GENMASK_32 for AARCH32,
and to GENMASK_64 for 64bit arch.

fixes arm-software/tf-issues#596

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>