Fix building GENERIC-MMCCAM on arm64

Since GENERIC includes quite a few drivers now, all MMC drivers should have
appropriate MMCCAM-related ifdefs and include opt_mmccam.h so that
those ifdefs are actually processed correctly.

Reviewed by:	manu
Approved by:	imp (mentor)
Differential Revision:	https://reviews.freebsd.org/D15569

sys/dev/mmc/host/dwmmc_rockchip.c

@@ -45,6 +45,8 @@ __FBSDID("$FreeBSD$");
 
 #include <dev/mmc/host/dwmmc_var.h>
 
+#include "opt_mmccam.h"
+
 enum RKTYPE {
 	RK2928 = 1,
 	RK3328,