configs/snps_arc*_defconfig: update linux version

With this commit we update Linux kernel version to 4.15 and
Linux headers version to 4.15.
Also patches for arc vdk are removed because these
changes were applied in Linux 4.15.

Signed-off-by: Evgeniy Didin <Evgeniy.Didin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: arc-buildroot@synopsys.com
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>