stm32l: workaround missing mass erase feature
stm32l: workaround missing mass erase feature

The STM32L15xx monitor does not implement the mass erase,
so we need to use the page erase feature and loop.

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

BUG=None
TEST=on Discovery, fill the flash with a pattern, then erase, write a
firmware image, read back the content of the flash, run the firmware.

Change-Id: Icf0e9812a5d491fea78472a0203ddbbc3e813b2f

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