Initial v1.2.1 for Portenta_H7

### Initial Releases v1.2.1

1. Initial coding to support **Portenta_H7 boards** such as Portenta_H7 Rev2 ABX00042, etc., using [**ArduinoCore-mbed mbed_portenta** core](https://github.com/arduino/ArduinoCore-mbed)
2. Permit up to 16 super-long-time, super-accurate ISR-based timers to avoid being blocked
3. Using cpp code besides Impl.h code to use if Multiple-Definition linker error.
4. Bump version to v1.2.1 to sync with [STM32_TimerInterrupt library](https://github.com/khoih-prog/STM32_TimerInterrupt)

Author: khoih-prog

README.md

@@ -66,7 +66,7 @@
 
 ### Features
 
-This library enables you to use Interrupt from Hardware Timers on an STM32-based board, such as STM32F/L/H/G/WB/MP1. 
+This library enables you to use Interrupt from Hardware Timers on an **STM32H7-based Portenta_H7** board. 
 
 As **Hardware Timers are rare, and very precious assets** of any board, this library now enables you to use up to **16 ISR-based Timers, while consuming only 1 Hardware Timer**. Timers' interval is very long (**ulong millisecs**).
 
@@ -814,7 +814,7 @@ void setup()
     Serial.println(F("Can't set ITimer correctly. Select another freq. or interval"));
 
   // Just to demonstrate, don't use too many ISR Timers if not absolutely necessary
-  // You can use up to 16 timer for each STM32_ISR_Timer
+  // You can use up to 16 timer for each Portenta_H7_ISR_Timer
   for (uint16_t i = 0; i < NUMBER_ISR_TIMERS; i++)
   {
 #if USE_COMPLEX_STRUCT

examples/ISR_16_Timers_Array_Complex/ISR_16_Timers_Array_Complex.ino

@@ -347,7 +347,7 @@ void setup()
     Serial.println(F("Can't set ITimer correctly. Select another freq. or interval"));
 
   // Just to demonstrate, don't use too many ISR Timers if not absolutely necessary
-  // You can use up to 16 timer for each STM32_ISR_Timer
+  // You can use up to 16 timer for each Portenta_H7_ISR_Timer
   for (uint16_t i = 0; i < NUMBER_ISR_TIMERS; i++)
   {
 #if USE_COMPLEX_STRUCT