mmc: sdhci-bcm-kona: Use sdhci_pltfm_unregister instead of open coded

This avoid duplicated implementation and also fixes missing iounmap() and
release_mem_region() calls in sdhci_bcm_kona_remove().  sdhci_pltfm_init()
calls request_mem_region() and ioremap(), thus we need to call the
corresponding iounmap() and release_mem_region() calls in
sdhci_bcm_kona_remove().

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Chris Ball <cjb@laptop.org>