Add OS_SDK_ZEPHYR definition to LVGL compile defines

[dberlin]

This enables us to use it in NXP's PXP accelerator (and later VGlite) support in LVGL.

(LVGL supports PXP, but only for freertos right now)

For a little more detail, Zephyr PXP support in LVGL requires two things:

1. For thread safety, LVGL uses a semaphore to tell when pxp completion happens, and so we need to use a zephyr semaphore to do that. See https://github.com/lvgl/lvgl/blob/master/src/draw/nxp/pxp/lv_pxp_osa.c for what i mean
2. Connecting the interrupt to the LVGL PXP Interrupt handler

To do #1, we need a define we can ifdef off in LVGL. The current FreeRTOS one is named OS_SDK_FREE_RTOS, so this introduces OS_SDK_ZEPHYR.

The rest of the patches for #1 go upstream, and are small (see zephyrproject-rtos/lvgl@zephyr...dberlin:lvgl:zephyr)

If zephyr is going to stick at 8.x for a bit, i can send backport patches as well

For #2, i'll send a patch to do the irq connect in lvgl_init if pxp support is enabled.

Note: All of this is unrelated to the current zephyr DMA driver and ELCDIF driver for PXP (which are related to doing hardware accelerated rotation for the display)

[thedjnK]

Why can't it use __ZEPHYR__ ?

[dberlin]

Why can't it use __ZEPHYR__ ?

I wasn't aware it existed :)
If it's meant for this purpose, happy to use it!

[faxe1008]

@dberlin
I raised the PR with your changes upstream, lvgl/lvgl#5838 since I could not find a PR of yours and I'd love to make this in.