stm32: Update keyboard_scan to use run-time configuration
stm32: Update keyboard_scan to use run-time configuration

At present the keyboard scan parameters are hard-coded, so changing them
requires a new EC image. This can be problematic if we want to adjust
the behavior of keyboard scanning since we must send an EC update.

Change stm32's keyboard scan to use run-time config. All parameters
and behavior should remain the same with this change.

The configuration is defined by ec_commands.h since we intend to create
a command to allow access to it. It does not seem worth defining a
separate structure within keyboard_scan at present, although if we
add a new version of the command in the future then we may want to
separate these.

BUG=chrome-os-partner:12179
BRANCH=snow
TEST=manual
Boot snow into U-Boot and Linux, try the keyboard, seeing that it seems
to work as before. This is a very subjective test.

Change-Id: Ie6160f1d73b983867b96f8ccb421853f6ec99524
Signed-off-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/34633
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>

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