Documentation/release: Add some of the things we added since 4.8.1

From here on, changes should directly touch the release notes, but these
are notable, too.

Change-Id: I602d67f8dd38391663094212cdb4609cdad458ee
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/26886
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>

Documentation/releases/coreboot-4.9-relnotes.md

@@ -16,6 +16,13 @@ before the release, so those do not need to be added.
 General changes
 ---------------
 
+* Various code cleanups
+ * Removed `device_t` in favor of `struct device*` in ramstage code
+ * Improve adherence to coding style
+* Expand use of the postcar stage
+* Add bootblock compression capability: on systems that copy the bootblock
+  from very slow flash to ERAM, allow adding a stub that decompresses the
+  bootblock into ERAM to minimize the amount of flash reads
 
 Toolchain
 ---------