dts: nxp: imx95-ca55: add dts for SoC 15x15 variant

As i.MX 95 19x19 and i.MX 95 15x15 have different pinmux definitions,
keep common part in nxp_mimx95_a55.dtsi, and define separate dts file
for each variants. These include the common part and their respective
pinmux definitions.

Signed-off-by: Aziz Sellami <aziz.sellami@nxp.com>