thermal: armada: Add support for Armada AP806

The AP806 component is integrated in the Armada 8K and 7K lines of
processors.

The thermal sensor sample field on the status register is a signed
value. Extend armada_get_temp() and the driver structure to handle
signed values.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
[<miquel.raynal@free-electrons.com>: Changes when applying over the
previous patches, including the register names changes, also switched
the coefficients values to s64 instead of unsigned long to deal with
negative values and used do_div instead of the traditionnal '/']
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>