thermal: armada: Fix possible overflow in the Armada 380 thermal sensor formula

Update the coefficients so the calculation will not overrun the
unsigned long 32bits boundary

Signed-off-by: Nadav Haklai <nadavh@marvell.com>
Reviewed-by: Victor Axelrod <victora@marvell.com>
Reviewed-by: Neta Zur Hershkovits <neta@marvell.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>