Add atomic flag for 16-bit ticks PIC24 (#488)

This change allows the PIC24 family of
16 bit processors to read the tick count
without a critical section when the tick
count is 16 bits.

Inspired from discussion - https://forums.freertos.org/t/xtaskgettickcount-with-critical-section-on-16-bit-mcu/14860/5
diff --git a/portable/MPLAB/PIC24_dsPIC/portmacro.h b/portable/MPLAB/PIC24_dsPIC/portmacro.h
index 223e493..0974bc3 100644
--- a/portable/MPLAB/PIC24_dsPIC/portmacro.h
+++ b/portable/MPLAB/PIC24_dsPIC/portmacro.h
@@ -59,6 +59,9 @@
 #if( configUSE_16_BIT_TICKS == 1 )

 	typedef uint16_t TickType_t;

 	#define portMAX_DELAY ( TickType_t ) 0xffff

+/* 16-bit tick type on a 16-bit architecture, so reads of the tick count do

+ * not need to be guarded with a critical section. */

+	#define portTICK_TYPE_IS_ATOMIC 1

 #else

 	typedef uint32_t TickType_t;

 	#define portMAX_DELAY ( TickType_t ) 0xffffffffUL