arm: zynq: slcr: Use read-modify-write for register writes

zynq_slcr_cpu_start/stop() ignored the current register state when
writing to a register. Fixing this by implementing proper
read-modify-write.
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm/mach-zynq/slcr.c

@@ -61,11 +61,11 @@ void zynq_slcr_system_reset(void)
  */
 void zynq_slcr_cpu_start(int cpu)
 {
-	/* enable CPUn */
-	writel(SLCR_A9_CPU_CLKSTOP << cpu,
-	       zynq_slcr_base + SLCR_A9_CPU_RST_CTRL_OFFSET);
-	/* enable CLK for CPUn */
-	writel(0x0 << cpu, zynq_slcr_base + SLCR_A9_CPU_RST_CTRL_OFFSET);
+	u32 reg = readl(zynq_slcr_base + SLCR_A9_CPU_RST_CTRL_OFFSET);
+	reg &= ~(SLCR_A9_CPU_RST << cpu);
+	writel(reg, zynq_slcr_base + SLCR_A9_CPU_RST_CTRL_OFFSET);
+	reg &= ~(SLCR_A9_CPU_CLKSTOP << cpu);
+	writel(reg, zynq_slcr_base + SLCR_A9_CPU_RST_CTRL_OFFSET);
 }
 
 /**
@@ -74,9 +74,9 @@ void zynq_slcr_cpu_start(int cpu)
  */
 void zynq_slcr_cpu_stop(int cpu)
 {
-	/* stop CLK and reset CPUn */
-	writel((SLCR_A9_CPU_CLKSTOP | SLCR_A9_CPU_RST) << cpu,
-	       zynq_slcr_base + SLCR_A9_CPU_RST_CTRL_OFFSET);
+	u32 reg = readl(zynq_slcr_base + SLCR_A9_CPU_RST_CTRL_OFFSET);
+	reg |= (SLCR_A9_CPU_CLKSTOP | SLCR_A9_CPU_RST) << cpu;
+	writel(reg, zynq_slcr_base + SLCR_A9_CPU_RST_CTRL_OFFSET);
 }
 
 /**