mmc: core: Allow building PWRSEQ_SD8787 with LIBERTAS_SDIO

The sd8686 "libertas" SDIO adapter's power is controlled with WLAN_RST
and WLAN_PD pins -- pretty much the same way as sd8787. Allow building
the power sequencing driver along with the libertas Wi-Fi driver.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>