ec/google/chromeec: provide reboot function

Provide a common function to issue reboot commands to the EC.
Expose that function for external use and use it internal to
the module.

BUG=b:35580805

Change-Id: I1458bd7119b0df626a043ff3806c15ffb5446c9a
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/19573
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>

src/ec/google/chromeec/ec.h

@@ -57,6 +57,10 @@ int google_chromeec_vstore_info(uint32_t *locked);
 int google_chromeec_vstore_read(int slot, uint8_t *data);
 int google_chromeec_vstore_write(int slot, uint8_t *data, size_t size);
 
+/* Issue reboot command to EC with specified type and flags. Returns 0 on
+   success, < 0 otherwise. */
+int google_chromeec_reboot(int dev_idx, enum ec_reboot_cmd type, uint8_t flags);
+
 /* MEC uses 0x800/0x804 as register/index pair, thus an 8-byte resource. */
 #define MEC_EMI_BASE		0x800
 #define MEC_EMI_SIZE		8