ram: ast2600: Improve ddr4 timing and signal quality

Adjust the following settings to get better timing and signal quality.

1. write DQS/DQ delay
- 1e6e2304[0]
- 1e6e2304[15:8]

2. read DQS/DQ delay
- 0x1e6e0298[0]
- 0x1e6e0298[15:8]

3. CLK/CA timing
- 0x1e6e01a8[31]

4. Read and write termination
- change RTT_ROM from 40 ohm to 48 ohm (MR1[10:8])
- change RTT_PARK from disable to 48 ohm (MR5[8:6])
- change RTT_WR from 120 ohm to disable (MR2[11:9])
- change PHY ODT from 40 ohm to 80 ohm (0x1e6e0130[10:8])

Note1: Both DDR-PHY and DDR controller have their own registers for DDR4
Mode Registers (MR0~MR6).  This patch introduces macros to synchronize
the MR value on both sides.

Note2: the waveform meansurement can be found in item #21 of Aspeed
AST26x0 Application note (AP note).

Review-by: Ryan Chen <ryan_chen@aspeedtech.com>
Signed-off-by: Dylan Hung <dylan_hung@aspeedtech.com>