boards: nxp: imx95_evk_15x15: add i.MX 95 15x15 LPDDR4x EVK variant

Add initial support for i.MX 95 15x15 LPDDR4x EVK board. This board
uses the i.MX 95 15x15 SoC that shares many similarities to the
already supported i.MX 95 19x19 SoC used for the i.MX 95 19x19
LPDDR5 EVK.

This enables Zephyr to boot and run on the i.MX 95 15x15 EVK and
provides a foundation for further peripheral enablement and
application development.

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