Merge tag 'u-boot-atmel-fixes-2021.04-a' of https://gitlab.denx.de/u-boot/custodians/u-boot-atmel

First set of u-boot-atmel fixes for 2021.04 cycle:

This small PR includes just two fixes but very important: one revert in
the clk subsystem which fixes the boot on many old boards
(sama5d2_xplained, sama5d4_xplained), which currently crash at boot; and
one small fix related to debug serial on sama7g5ek board.