arm: socfpga: rst: add register definition for cold reset

This adds a define for the bit in rstmgr's ctrl regiser that issues
a cold reset (we had a define for the warm reset bit only) in preparation
for a proper sysrese driver.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Series changes: 2
- separate this patch to the register descriptions from the actual
  sysreset driver patch