core: add serial abstraction
core: add serial abstraction

Adds an abstraction for a serial device.

Reviewed-by: etienne carriere <f21bc5b8f3ea1962eb45af359f1abea6578cab94@linaro.org>
Reviewed-by: David Brown <0af22db655f3ade5845314d695c1f4910314b0cb@linaro.org>
Signed-off-by: Jens Wiklander <7706914404370d7502c27a0eff493dcf491feb51@linaro.org>

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