hal qspi test: adjust min frequency for EFM32GG11

according to the driver limitations

Author: maciejbocianski

TESTS/mbed_hal/qspi/flash_configs/SiliconLabs/EFM32GG11_STK3701/flash_config.h

@@ -23,4 +23,6 @@
 #define QSPI_SECTOR_COUNT                       1024 // for MX25R3235F
 #endif
 
+#define QSPI_MIN_FREQUENCY  2000000
+
 #endif // MBED_QSPI_FLASH_CONFIG_H

TESTS/mbed_hal/qspi/main.cpp

@@ -42,7 +42,9 @@ using namespace utest::v1;
 
 
 
+#ifndef QSPI_MIN_FREQUENCY
 #define QSPI_MIN_FREQUENCY  1000000
+#endif
 
 // max write size is usually page size
 #define DATA_SIZE_256  (QSPI_PAGE_SIZE)