README.md: add pllmf exception for gd32a50x

gd32a50x have pllmf calculate error, explain how to fix it.

Signed-off-by: YuLong Yao <[email protected]>

Author: feilongfl

README.md

@@ -125,3 +125,15 @@ conflict resolution. See below list with the proposed solution:
 
   - i2c have different implement than current gd32 i2c driver. no need to patch
     upper i2c speed requirement.
+
+- gd32a50x
+
+  - `SystemCoreClockUpdate` function contain an `pllmf` calculate error.  
+    Fix by change `0xFU` to `RCU_CFG0_PLLMF`:
+    ``` diff
+      /* PLL multiplication factor */
+      pllmf = GET_BITS(RCU_CFG0, 18, 21);
+      pllmf += ((RCU_CFG0 & RCU_CFG0_PLLMF_4) ? 15U : 0U);
+    - pllmf += ((0xFU == (RCU_CFG0 & RCU_CFG0_PLLMF)) ? 1U : 2U);
+    + pllmf += ((RCU_CFG0_PLLMF == (RCU_CFG0 & RCU_CFG0_PLLMF)) ? 1U : 2U);
+    ```