rockchip: convert to use ROCKCHIP_BOOT_MODE_REG for fastboot tag

The fastboot_set_reboot_flag() update a TAG into a register
for next boot, use the common macro for the register so that
we can re-use the function for different SoCs.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/arch/arm/mach-rockchip/rk3128-board.c b/arch/arm/mach-rockchip/rk3128-board.c
index 0945829..636ee65 100644
--- a/arch/arm/mach-rockchip/rk3128-board.c
+++ b/arch/arm/mach-rockchip/rk3128-board.c
@@ -111,12 +111,9 @@
 #if CONFIG_IS_ENABLED(FASTBOOT)
 int fastboot_set_reboot_flag(void)
 {
-	struct rk3128_grf *grf;
-
 	printf("Setting reboot to fastboot flag ...\n");
-	grf = syscon_get_first_range(ROCKCHIP_SYSCON_GRF);
 	/* Set boot mode to fastboot */
-	writel(BOOT_FASTBOOT, &grf->os_reg[0]);
+	writel(BOOT_FASTBOOT, CONFIG_ROCKCHIP_BOOT_MODE_REG);
 
 	return 0;
 }
diff --git a/arch/arm/mach-rockchip/rk322x-board.c b/arch/arm/mach-rockchip/rk322x-board.c
index e7a1e54..c0a5376 100644
--- a/arch/arm/mach-rockchip/rk322x-board.c
+++ b/arch/arm/mach-rockchip/rk322x-board.c
@@ -115,12 +115,9 @@
 #if CONFIG_IS_ENABLED(FASTBOOT)
 int fastboot_set_reboot_flag(void)
 {
-	struct rk322x_grf *grf;
-
 	printf("Setting reboot to fastboot flag ...\n");
-	grf = syscon_get_first_range(ROCKCHIP_SYSCON_GRF);
 	/* Set boot mode to fastboot */
-	writel(BOOT_FASTBOOT, &grf->os_reg[0]);
+	writel(BOOT_FASTBOOT, CONFIG_ROCKCHIP_BOOT_MODE_REG);
 
 	return 0;
 }