thermal: armada: Clarify control registers accesses

Bindings were incomplete for a long time by only exposing one of the two
available control registers. To ease the migration to the full bindings
(already in use for the Armada 375 SoC), rename the pointers for
clarification. This way, it will only be needed to add another pointer
to access the other control register when the time comes.

This avoids dangerous situations where the offset 0 of the control
area can be either one register or the other depending on the bindings
used. After this change, device trees of other SoCs could be migrated to
the "full" bindings if they may benefit from features from the
unaccessible register, without any change in the driver.

Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Reviewed-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>