mips: mtmips: rewrite lowlevel codes of mt7628

mips: mtmips: rewrite lowlevel codes of mt7628

This patch rewrites the mtmips architecture with the following changes:

1. Move MT7628 soc parts into a subfolder.
2. Lock parts of D-Cache as temporary stack.
3. Reimplement DDR initialization in C language.
4. Reimplement DDR calibration in a clear logic.
5. Add full support for auto size detection for DDR1 and DDR2.
6. Use accurate CPU clock depending on the input xtal frequency for timer
   and delay functions.

Note:

print_cpuinfo() has incompatible parts with MT7620 so it's moved into
mt7628 subfolder.

Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>