arm64: Introduce config for S32

Add configuration option for the NXP S32 platform family in
Kconfig.platforms. For starters, the only SoC supported will be Treerunner
(S32V234), with a single execution target: the S32V234-EVB (rev 29288)
board.

Signed-off-by: Mihaela Martinas <Mihaela.Martinas@freescale.com>
Signed-off-by: Stoica Cosmin-Stefan <cosmin.stoica@nxp.com>
Signed-off-by: Stefan-Gabriel Mirea <stefan-gabriel.mirea@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>