soc/core: simplify/cleanup HyperRAM core

soc/core: simplify/cleanup HyperRAM core
- rename core to hyperbus.
- change layout (cs_n with variable length instead of cs0_n, cs1_n).
- use DifferentialOutput when differential clock is used.
- add test (python3 -m unittest test.test_hyperbus).

Usage example:
from litex.soc.cores.hyperbus import HyperRAM
self.submodules.hyperram = HyperRAM(platform.request("hyperram"))
self.add_wb_slave(mem_decoder(self.mem_map["hyperram"]), self.hyperram.bus)
self.add_memory_region("hyperram", self.mem_map["hyperram"], 8*1024*1024)