Like Bill said, "man up" can be seen as as a stupid phrase. However, it does mean to stop feeling sorry for yourself and be an adult (male) instead of a child or baby. So, "woman up" would mean the same thing, addressed to a woman: don't be a child, be an adult (woman). Depending on tone, it could be a rebuke, or it could be friendly and helpful like you said--like "cheer up."

woman up is an exact copy of "man up" which is an american phrase to mean "be brave, single minded, lacking in empathy, kindness, sympathy, artistic merit" etc. The idea is that there are natural roles for individuals based on their gender. "woman up" then can be seen as one of three things 1) An ironic comment on the dumb idiots who say "man up" 2) A suggestion to be more like a man but given to a woman 3) A suggestion to behave more a like woman in the view of the speaker about what they think a woman should be like (place you cultural stereotype here)

"woman up" does not mean the same as "cheer up", see Bill's excellent answer :)

I've never heard 'woman up' being used! 'man up' is common and means to be brave, so it would make sense that 'woman up' could refer to a more feminine trait!