Both TTB index and address used to fill that entry are derived from
the same variable 'addr' which requires shifting right and left by 20
and somewhat confusing.

Split the counter used to iterate over elements of  TTB into a
separate variable to make this code a bit easier to read.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>