stm32: add wakeup from serial port as a debug feature
stm32: add wakeup from serial port as a debug feature

When STOP mode is activated, the UART is not able to wakeup the EC when
sleeping, preventing to enter commands on the EC serial console.
Allow to switch the UART RX line as a GPIO connected to EXTINT10 to
wakeup the system on incoming character.

This is just a debug feature since EXTINT10 is normally used to scan the
keyboard.

Signed-off-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

BUG=chrome-os-partner:8866
TEST=on Snow, enable CONFIG_FORCE_CONSOLE_RESUME at build time and type
"sleepmask 0" on the EC console, see I can get the serial console back
by typing a character on the serial console.

Change-Id: I936cbf13707ef8cde277f1053a4d35d23ff06511
Reviewed-on: https://gerrit.chromium.org/gerrit/29776
Reviewed-by: David Hendricks <3b0deacc4b5357f519084f1ea34bd42bf5d34545@chromium.org>
Commit-Ready: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Tested-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

alterapraxisptyltd/chromium-ec,thehobn/ec,coreboot/chrome-ec,gelraen/cros-ec,thehobn/ec,md5555/ec,akappy7/ChromeOS_EC_LED_Diagnostics,mtk09422/chromiumos-platform-ec,mtk09422/chromiumos-platform-ec,gelraen/cros-ec,thehobn/ec,eatbyte/chromium-ec,fourier49/BIZ_EC,longsleep/ec,fourier49/BIZ_EC,gelraen/cros-ec,fourier49/BZ_DEV_EC,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,mtk09422/chromiumos-platform-ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,thehobn/ec,fourier49/BZ_DEV_EC,md5555/ec,alterapraxisptyltd/chromium-ec,coreboot/chrome-ec,longsleep/ec,alterapraxisptyltd/chromium-ec,eatbyte/chromium-ec,longsleep/ec,md5555/ec,alterapraxisptyltd/chromium-ec,fourier49/BZ_DEV_EC,eatbyte/chromium-ec,gelraen/cros-ec,coreboot/chrome-ec,coreboot/chrome-ec,longsleep/ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,md5555/ec,eatbyte/chromium-ec,fourier49/BIZ_EC,fourier49/BIZ_EC,mtk09422/chromiumos-platform-ec