Is VHDL for digital circuits only?
Not in its base specification. 
VHDL stands for V(HSIC) H(ardware) D(escriptive) L(anguage) with VHSIC standing for Very High Speed Integrated Circuit.

There is an analogue extension to VHDL which is VHDL-AMS 

https://en.wikipedia.org/wiki/VHDL-AMS


  VHDL-AMS is a derivative of the hardware description language VHDL
  (IEEE standard 1076-1993). It includes analog and mixed-signal
  extensions (AMS) in order to define the behavior of analog and
  mixed-signal systems (IEEE 1076.1-1999).
  
  The VHDL-AMS standard was created with the intent of enabling
  designers of analog and mixed signal systems and integrated circuits
  to create and use modules that encapsulate high-level behavioral
  descriptions as well as structural descriptions of systems and
  components.[1]
  
  VHDL-AMS is an industry standard modeling language for mixed signal
  circuits. It provides both continuous-time and event-driven modeling
  semantics, and so is suitable for analog, digital, and mixed
  analog/digital circuits. It is particularly well suited for
  verification of very complex analog, mixed-signal and radio frequency
  integrated circuits.