rtc: st-lpc: get IRQ via platform_get_irq()

rtc: st-lpc: get IRQ via platform_get_irq()

Replace irq_of_parse_and_map() with platform_get_irq(), which resolves
the interrupt from pdev->dev.of_node directly and returns a positive IRQ
or a negative errno (it never returns 0). Propagate the error on failure
instead of the previous open-coded "IRQ missing or invalid" / -EINVAL
message, so -EPROBE_DEFER is handled correctly. The rtc->irq field is a
signed short, so the negative error code is preserved.

np is still used by of_property_read_u32() for "st,lpc-mode", so it is
not removed.

Built for ARM (multi_v7_defconfig + CONFIG_RTC_DRV_ST_LPC) with LLVM=1;
drivers/rtc/rtc-st-lpc.o compiles cleanly.

Assisted-by: opencode:hy3-free
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Link: https://patch.msgid.link/20260714002259.1392655-1-rosenp@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>