defconfig: sa8155: Unset the STD partition

Remove the suspend-to-disk partition name as it is being added to the
kernel 'resume' command line by the bootloader.

Change-Id: Ied889e403563d9998bc3f9f71d07698bce53a44c
Signed-off-by: Shreyas K K <shrekk@codeaurora.org>