Update PersistentStore api (#11538)

- Clean up the API to use a `static` class instance to adhere to Marlin convention
- Add `const` position data access for read/write
- Add Storage capacity to the interface

Marlin/src/HAL/HAL_STM32F4/persistent_store_eeprom.cpp

@@ -29,13 +29,10 @@
 
 #if ENABLED(EEPROM_SETTINGS)
 
-namespace HAL {
-namespace PersistentStore {
+bool PersistentStore::access_start() { return true; }
+bool PersistentStore::access_finish() { return true; }
 
-bool access_start() { return true; }
-bool access_finish() { return true; }
-
-bool write_data(int &pos, const uint8_t *value, uint16_t size, uint16_t *crc) {
+bool PersistentStore::write_data(int &pos, const uint8_t *value, size_t size, uint16_t *crc) {
   while (size--) {
     uint8_t * const p = (uint8_t * const)pos;
     uint8_t v = *value;
@@ -56,7 +53,7 @@ bool write_data(int &pos, const uint8_t *value, uint16_t size, uint16_t *crc) {
   return false;
 }
 
-bool read_data(int &pos, uint8_t* value, uint16_t size, uint16_t *crc, const bool writing) {
+bool PersistentStore::read_data(int &pos, uint8_t* value, size_t size, uint16_t *crc, const bool writing) {
   do {
     uint8_t c = eeprom_read_byte((unsigned char*)pos);
     if (writing) *value = c;
@@ -67,8 +64,21 @@ bool read_data(int &pos, uint8_t* value, uint16_t size, uint16_t *crc, const boo
   return false;
 }
 
-} // PersistentStore
-} // HAL
+bool PersistentStore::write_data(const int pos, uint8_t* value, size_t size) {
+  int data_pos = pos;
+  uint16_t crc = 0;
+  return write_data(data_pos, value, size, &crc);
+}
+
+bool PersistentStore::read_data(const int pos, uint8_t* value, size_t size) {
+  int data_pos = pos;
+  uint16_t crc = 0;
+  return read_data(data_pos, value, size, &crc);
+}
+
+const size_t PersistentStore::capacity() {
+  return E2END + 1;
+}
 
 #endif // EEPROM_SETTINGS
 #endif // STM32F4 || STM32F4xx