configs/snps_arc*_defconfig: update u-boot version

configs/snps_arc*_defconfig: update u-boot version

With this commit we update u-boot version to 2018.05 for
Synopsys boards. U-boot version 2018.05 was released recently
and includes significant changes for ARC boards:
 * Fix for compile-time warning for AXS10x
 * Add support of platform-specific commands for HSDK
 * Add support for on-board SPI flash on HSDK

Signed-off-by: Evgeniy Didin <[email protected]>
Cc: Thomas Petazzoni <[email protected]>
Cc: [email protected]
Signed-off-by: Thomas Petazzoni <[email protected]>