Update README for i.MX6UL EVK
Update README for i.MX6UL EVK

Signed-off-by: Peng Fan <7351ee4254d3ae583dd8772c904897a231cf4948@gmail.com>
Reviewed-by: Joakim Bech <741f90e2a7f4d9afbad8bd50be20560f4cf38962@linaro.org>

matt2048/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,matt2048/optee_os,matt2048/optee_os,pascal-brand-st-dev/optee_os,matt2048/optee_os