"which software I need"

Because it's a standard 8051 core, any 8051 compiler/assembler can be used to generate the code.
Of course, this includes Keil.
Do read Keil Application Note 159: Downloading to Triscend E5 Devices
http://www.keil.com/appnotes/docs/apnt_159.asp

Keil uVision also has full debug support for the E5.

You will need Triscend's FastChip software to create the logic configuration, and download it (see the App Note for integrating this with uVision). It also provides a hardware-oriented debugger.

The free FastChip download has a limit on the size of logic it'll use; dunno what the price of the unrestricted version is these days.