modules: lvgl: provide alignment definition

For some systems, the display peripheral or GPU requires that buffers be
aligned on a boundary for optimal performance. We already align display
buffers using CONFIG_LV_ATTRIBUTE_MEM_ALIGN_SIZE, but some applications
will rely on the definition of LV_ATTRIBUTE_MEM_ALIGN. Provide this
definition so those buffers will be aligned as well.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>