tty: serial: Add linflexuart driver for S32V234

Introduce support for LINFlex driver, based on:
- the version of Freescale LPUART driver after commit b3e3bf2ef2c7 ("Merge
  4.0-rc7 into tty-next");
- commit abf1e0a98083 ("tty: serial: fsl_lpuart: lock port on console
  write").
In this basic version, the driver can be tested using initramfs and relies
on the clocks and pin muxing set up by U-Boot.

Remarks concerning the earlycon support:

- LinFlexD does not allow character transmissions in the INIT mode (see
  section 47.4.2.1 in the reference manual[1]). Therefore, a mutual
  exclusion between the first linflex_setup_watermark/linflex_set_termios
  executions and linflex_earlycon_putchar was employed and the characters
  normally sent to earlycon during initialization are kept in a buffer and
  sent afterwards.

- Empirically, character transmission is also forbidden within the last 1-2
  ms before entering the INIT mode, so we use an explicit timeout
  (PREINIT_DELAY) between linflex_earlycon_putchar and the first call to
  linflex_setup_watermark.

- U-Boot currently uses the UART FIFO mode, while this driver makes the
  transition to the buffer mode. Therefore, the earlycon putchar function
  matches the U-Boot behavior before initializations and the Linux behavior
  after.

[1] https://www.nxp.com/webapp/Download?colCode=S32V234RM

Signed-off-by: Stoica Cosmin-Stefan <[email protected]>
Signed-off-by: Adrian.Nitu <[email protected]>
Signed-off-by: Larisa Grigore <[email protected]>
Signed-off-by: Ana Nedelcu <[email protected]>
Signed-off-by: Mihaela Martinas <[email protected]>
Signed-off-by: Matthew Nunez <[email protected]>
[[email protected]: Reduced for upstreaming and implemented
                               earlycon support]
Signed-off-by: Stefan-Gabriel Mirea <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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