boards: arm: Configure FlexSPI QSPI flash on mimxrt1064_evk

Enables the FlexSPI NOR flash driver, configures the FlexSPI pins, and
updates the board documentation accordingly on the mimxrt1064_evk.

Note that this SoC has two FlexSPI instances: one instance has an
in-package QSPI flash used for XIP; the other instance has a board-level
QSPI flash used for storage, not XIP. This patch enables the flash
driver on the non-XIP flash only.

Tested with:
  - samples/subsys/fs/littlefs
  - samples/drivers/flash_shell

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>

boards: arm: Rename flexspi_qspi to flexspi_nor for mimxrt1064_evk

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>