memory: brcmstb_dpfe: add separate entry for compile test

Add separate entry for Broadcom STB DPFE driver, enabled by default on
ARCH_BRCMSTB.  This allows compile testing.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>