documentation: Add some text to describe DTB overlays to optee_design
documentation: Add some text to describe DTB overlays to optee_design

This patch adds some documentary text to optee_design.md describing how
OPTEE DTB overlay works.

Signed-off-by: Bryan O'Donoghue <165dc05575478af93ac2f97f67be6e1756010094@linaro.org>
Reviewed-by: Etienne Carriere <f21bc5b8f3ea1962eb45af359f1abea6578cab94@linaro.org>

pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os