defconfig: Disable CONFIG_DEVMEM and CONFIG_DEVKMEM

testDevMemSane & testDevkmemSane test cases are introduced
in latest Android-6.0 version of CTS. These testcases expect
mem and kmem nodes should not be present in /dev.

CRs-fixed: 907542
Change-Id: Icd798c885b6045e889ff720aa8655351d1376353
Signed-off-by: Swetha Chikkaboraiah <schikk@codeaurora.org>