stm32: Allow QSPI to work on STM32G4.

Adding a QSPI memory chip on a STM32G4 does not work due to some small
issues, which are fixed in this commit:

- Rename QUADSPI1_xxx alt-func names to QUADSPI_xxx, to match the static
  names used in `qspi.c`.

- Enable `mpu.h` macros on G4.

- Don't include I- and D-cache invalidation on G4.

Signed-off-by: Damien George <[email protected]>

Author: H-Grobben