reset: socfpga: no need to store modrst_offset

Since we can just add it to membase once, there is no need to store
modrst_offset separately, and to repeat the addition with every access.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Dinh Nguyen <dinguyen@opensource.altera.com>