update coreboot/dell to same rev as default

re-use the same patches, and drop the same patches.

this tree uses hell's special ddr2 fix, which we apply
for the dell latitude e6400.

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/coreboot/dell/target.cfg

@@ -1,4 +1,4 @@
 tree="dell"
 tree_depend="default"
 xtree="default"
-rev="d862695f5f432b5c78dada5f16c293a4c3f9fce6"
+rev="b6cbfa977f63d57d5d6b9e9f7c1cef30162f575a"