reset: socfpga: Update reset-socfpga to read the altr,modrst-offset property

In order for the Arria10 to be able to re-use the reset driver for SoCFPGA
Cyclone5/Arria5, we need to read the 'altr,modrst-offset' property from the
device tree entry. The 'altr,modrst-offset' property is the first register
into the reset manager that is used for bringing peripherals out of reset.

The driver assumes a modrst-offset of 0x10 in order to support legacy
Cyclone5/Arria5 hardware.

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>