Merge tag 'arc-fixes-for-2020.07-rc1' of https://gitlab.denx.de/u-boot/custodians/u-boot-arc

This is pretty minor set of changes mostly touching HSDK board:

 * Enable on-chip reset controller on HSDK
 * Add possibility to turn-on & off L2$ on more
   recent ARC HS processors.
 * AXI tunnel clock calculation on HSDK