serial: efi-stdio: Add simple_text_input_ex_protocol backend ...

EFI implementations differ in how simple_text_input_protocol treats
ctrl+alpha combinations. OVMF (used with QEMU) returns a Unicode control
character, the UEFI on the Dell Latitude 7490 on the other hand ignores
the ctrl completely and returns just the letter rendering utilities
like barebox edit unusable.

To fix this, the simple_text_input_ex_protocol can be leveraged as it
additionally provides the state of modifier keys. Extend efi-stdio to use
it where possible.

Cc: Michael Olbrich <mol@pengutronix.de>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>