ddr: altera: Ignore bit[7-4] for both seq2core & core2seq handshake in HPS

Bit[7-4] for both register seq2core and core2seq handshake in HPS are not
required for triggering DDR re-calibration or resetting EMIF. So, ignoring
these bits just for playing it safe.

Signed-off-by: Tien Fong Chee <tien.fong.chee@intel.com>