[Keyboard] Acheron Athena support (#16818)

Co-authored-by: Drashna Jaelre <drashna@live.com>

keyboards/acheron/athena/beta/mcuconf.h

@@ -0,0 +1,44 @@
+/* Copyright 2020 QMK
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include_next <mcuconf.h>
+#undef STM32_PLLM_VALUE
+#undef STM32_PLLN_VALUE
+#undef STM32_PLLP_VALUE
+#undef STM32_PLLQ_VALUE
+#undef STM32_PPRE1
+#undef STM32_PPRE2
+
+#define STM32_PLLM_VALUE                    4
+#define STM32_PLLN_VALUE                    168
+#define STM32_PLLP_VALUE                    4
+#define STM32_PLLQ_VALUE                    7
+#define STM32_PPRE1                         STM32_PPRE1_DIV2
+#define STM32_PPRE2                         STM32_PPRE2_DIV1
+
+#undef STM32_I2C_USE_I2C1
+#define STM32_I2C_USE_I2C1 TRUE
+
+#undef STM32_PWM_USE_ADVANCED
+#define STM32_PWM_USE_ADVANCED TRUE
+
+#undef STM32_PWM_USE_TIM3
+#define STM32_PWM_USE_TIM3 TRUE
+
+#undef STM32_PWM_USE_TIM1
+#define STM32_PWM_USE_TIM1 TRUE