Based on this rule, it looks like you can turn it face up:
701.31b Any time you have priority, you may turn a manifested permanent you control face up. This is a special action that doesn't use the stack (see rule 115.2b). To do this, show all players that the card representing that permanent is a creature card and what its mana cost is, pay that cost, then turn the permanent face up. The effect defining its characteristics while it was face down ends, and it regains its normal characteristics. If the card representing that permanent isn't a creature card or doesn't have a mana cost, it can't be turned face up this way.
While you may not have enough devotion, Purphoros ability doesn't do anything until after it has been turned face up. This means that while it is face down, it is still a creature card (regardless of devotion) and the rules allow any creature card to be turned face up
Based on this rule, it looks like you can turn it face up:
While you may not have enough devotion, Purphoros ability doesn't do anything until after it has been turned face up. This means that while it is face down, it is still a creature card (regardless of devotion) and the rules allow any creature card to be turned face up