Add GD32F30X TIMER7 definitions for soft UART

Introduce a GD32F30X-specific preprocessor branch in lib-gd32/src/softuart0/uart0.cpp that maps TIMERx to TIMER7 and sets RCU_TIMERx, TIMERx_IRQHandler, and TIMERx_IRQn accordingly. This enables the soft UART code to compile and use the correct timer/IRQ on GD32F30X devices.

Author: vanvught

lib-gd32/include/device/host/host.h

@@ -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