Support flash write protect on STM32L

This adds support for write protecting the RO code at boot, and the
entire flash on demand.

Implementation if WP# is not asserted is currently a little different
than STM32F and LM4; RO is still protected at boot if ro_at_boot, but
can be unprotected and the change will commit on the next reboot.
This saves the bank of flash which we use for pstate on LM4 and
STM32F.  I think I can use one of the unused option bits (WRP2 bit 0)
to hold the RO-at-boot flag, in which case I can more closely match
the behavior of the other chips, but I'd like to do that (or give up
and implement pstate) in a separate CL so it's clearer what I'm doing.

BUG=chrome-os-partner:15613
BRANCH=none
TEST=manual

- flashinfo -> (no flags)
- enable WP (via screw or dut-control)
- flashinfo -> wp_gpio_asserted
- flashwp enable
- flashinfo -> wp_gpio_asserted ro_at_boot
- flashwp now
- flashinfo -> wp_gpio_asserted ro_at_boot all_now
- flashwp disable -> fails
- flashinfo -> wp_gpio_asserted ro_at_boot all_now
- flasherase 0x1fc00 0x400 -> fails
- reboot
- flashinfo -> wp_gpio_asserted ro_at_boot ro_now
- flasherase 0xfc00 0x400 -> fails
- flasherase 0x1fc00 0x400 -> succeeds
- disable WP (via screw or dut-control)
- reboot
- flashinfo -> ro_at_boot ro_now
- flashwp disable
- flashinfo -> ro_now
- reboot
- flashinfo -> (no flags)
- flasherase 0xfc00 0x400 -> succeeds
- flasherase 0x1fc00 0x400 -> succeeds

Change-Id: Id1b6b099a44a1985a5ab9387feb882a8f26e3aa1
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/55594

include/flash.h

@@ -70,6 +70,17 @@ int flash_physical_erase(int offset, int size);
  */
 int flash_physical_get_protect(int bank);
 
+/**
+ * Force reload of flash protection bits.
+ *
+ * Some EC architectures (STM32L) only load the bits from option bytes at
+ * power-on reset or via a special command.  This issues that command if
+ * possible, which triggers a power-on reboot.
+ *
+ * Only returns (with EC_ERROR_ACCESS_DENIED) if the command is locked.
+ */
+int flash_physical_force_reload(void);
+
 /*****************************************************************************/
 /* High-level interface for use by other modules. */