Support GD32F30X TIMER7 in uart0

Add a preprocessor branch for GD32F30X in lib-gd32/src/softuart0/uart0.cpp that maps TIMERx to TIMER7 (TIMER7, RCU_TIMER7, TIMER7_UP_IRQHandler, TIMER7_UP_IRQn). This enables soft UART timer configuration for the GD32F30X variant.

Author: vanvught

lib-gd32/src/softuart0/uart0.cpp

@@ -32,6 +32,11 @@
 #define RCU_TIMERx        RCU_TIMER15
 #define TIMERx_IRQHandler TIMER15_IRQHandler
 #define TIMERx_IRQn       TIMER15_IRQn
+#elif defined(GD32F30X)
+#define TIMERx            TIMER7
+#define RCU_TIMERx        RCU_TIMER7
+#define TIMERx_IRQHandler TIMER7_UP_IRQHandler
+#define TIMERx_IRQn       TIMER7_UP_IRQn
 #else
 #define TIMERx            TIMER9
 #define RCU_TIMERx        RCU_TIMER9