What value does Foucault give to dreams and the unconscious?

Your question is asking about the relationship between Foucault and psychoanalysis, more specifically Sigmund Freud, the father of psychoanalysis, and his theories on the unconscious and dreaming. While at first one can say of course he was influenced by psychoanalysis, upon a deeper examination it is important to notice Foucault's critique of it.

Foucault wrote an introduction to Binswanger's paper in 1954 where he said that dreams are the beginning of the earth and where one could understand the beginning of one's own heart. We see from this introduction that he was deeply influenced by Freud's work on dreaming.

Later Foucault departs from Marxist and Psychoanalytic thought in his critique of modern society in his book The History of Sexuality. He believed that psychoanalysis main power came from its focus on the unconscious. He criticized Freuds focus on sexuality and its power believing that overly focusing on sexuality within every interpretation was actually giving into desire and reinforcing traditional forms of power and hegemony.

Instead Foucault would prefer that we use the unconscious and dreams as a source for resistance, not to give into the dominant powers of sexuality and unconscious desires. He argued that in order to change dominant Western power systems we must develop a philosophy of resistance. 

See:

Foucault, Michel. The History of Sexuality An Introduction. New York: Vintage. 1990. ISBN: 0679724699.

Miller, James (1993). The Passion of Michel Foucault. New York City: Simon & Schuster. ISBN 978-0-674-00157-2.