arm: Implement read_mpidr on armv7

Implement read_mpidr() on armv7 to make use of it in generic
code that compiles on both armv7 and armv8.

Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/arm/include/asm/system.h

@@ -394,6 +394,15 @@ void switch_to_hypervisor_ret(void);
 #define wfi()
 #endif
 
+static inline unsigned long read_mpidr(void)
+{
+	unsigned long val;
+
+	asm volatile("mrc p15, 0, %0, c0, c0, 5" : "=r" (val));
+
+	return val;
+}
+
 static inline unsigned long get_cpsr(void)
 {
 	unsigned long cpsr;