drivers: memc: flexspi: Update init priority

Update flexspi driver init priority to be the
memc driver init priority. This fixes a bug where
the flexspi flash drivers on i.MX RT platforms were
being initialized before the flexspi memc driver.
Since those flash drivers depend on the flexspi
spi bus controller being initialized, the flash drivers
would fail to initialize and cause runtime failures.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>