soc/rockchip/rk3288: Add fall through comment

Judging from the state machine on page 281 of the Rockchip RK3288
Technical Reference Manual (Rev 1.0 - Jun 2015), the fall through
from the INIT_MEM -> CONF states is intentional, since that is the
only way to get to the ACCESS state. Add a comment to explain this.

Change-Id: I1d0cfea07211c54d6a906f5a7481c2c760f8ef0d
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Found-by: Coverity CID 1291959
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34296
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/soc/rockchip/rk3288/sdram.c

@@ -908,6 +908,7 @@ static void move_to_access_state(u32 chnum)
 			while ((read32(&ddr_pctl_regs->stat) & PCTL_STAT_MSK)
 				!= CONF)
 				;
+			/* fall through - enter config next to get to access state */
 		case CONF:
 			write32(&ddr_pctl_regs->sctl, GO_STATE);
 			while ((read32(&ddr_pctl_regs->stat) & PCTL_STAT_MSK)