Convert illegal constexpr to static const pointers

We cannot get a constexpr definition of the register addresses, and any
reinterpret cast is currently illegal for a constexpr in c++17.

Change the SPI0, TIFR and TIMSK to volatile const pointers instead.

SPI0 has volatile members instead of marking the entire struct as
volatile, which is probably not a good idea as it technically drops the
volatile from the original pointer.

src/hal/spi.h

@@ -3,7 +3,10 @@
 #include <inttypes.h>
 #include "gpio.h"
 
-#define SPI0 ((hal::spi::SPI_TypeDef *)&SPCR)
+#ifdef __AVR__
+#define SPI0 ((hal::spi::SPI_TypeDef *const) & SPCR)
+#endif
+
 namespace hal {
 
 /// SPI interface
@@ -30,9 +33,9 @@ void Init(SPI_TypeDef *const hspi, SPI_InitTypeDef *const conf);
 uint8_t TxRx(SPI_TypeDef *hspi, uint8_t val);
 
 #ifdef __AVR__
-constexpr SPI_TypeDef *TmcSpiBus = SPI0;
+static SPI_TypeDef *const TmcSpiBus = SPI0;
 #else
-constexpr SPI_TypeDef *TmcSpiBus = nullptr;
+static SPI_TypeDef *const TmcSpiBus = nullptr;
 #endif
 
 }

src/hal/timers.h

@@ -20,8 +20,8 @@ struct Tim8bit_TypeDef {
     volatile uint8_t OCRxB;
 };
 
-constexpr volatile uint8_t *TIFR = &TIFR0;
-constexpr volatile uint8_t *TIMSK = &TIMSK0;
+static volatile uint8_t *const TIFR = &TIFR0;
+static volatile uint8_t *const TIMSK = &TIMSK0;
 
 struct Tim8bit_CTC_config {
     uint8_t cs : 3; ///clock source as per datasheet. It is not consistent between timer types

src/hal/tmc2130.h

@@ -18,7 +18,7 @@ enum MotorMode : uint8_t {
 };
 
 struct MotorParams {
-    const hal::spi::SPI_TypeDef *spi;
+    const hal::spi::SPI_TypeDef *const spi;
     uint8_t idx; ///< SHR16 index
     bool dirOn; ///< forward direction
     gpio::GPIO_pin csPin; ///< CS pin