defconfig: arm: 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: I162fb4b2f7eeeace752112bd919be1712dc2cb85
Signed-off-by: Venkatesh Yadav Abbarapu <vabbar@codeaurora.org>