clk: add config(5) entry for rk_clk_mmc.c

This is needed in order to unbreak the build on armv7.

Reported by:	Jenkins
Fixes:		e32b082ba2 ("clk: rockchip: Add the MMC phase clock")

sys/arm/rockchip/files.rk32xx

@@ -17,6 +17,7 @@ dev/clk/rockchip/rk_clk_armclk.c	standard
 dev/clk//rockchip/rk_clk_composite.c	standard
 dev/clk//rockchip/rk_clk_fract.c	standard
 dev/clk//rockchip/rk_clk_gate.c		standard
+dev/clk/rockchip/rk_clk_mmc.c		standard
 dev/clk//rockchip/rk_clk_mux.c		standard
 dev/clk//rockchip/rk_clk_pll.c		standard
 dev/clk//rockchip/rk_cru.c		standard