soc: move RTC initialization out of IRAM

Saves approximately 870 bytes of IRAM.

components/soc/linker.lf

@@ -3,8 +3,7 @@ archive: libsoc.a
 entries:
     cpu_util (noflash_text)
     rtc_clk (noflash)
-    rtc_clk_init (noflash_text)
-    rtc_init (noflash_text)
+    rtc_init:rtc_vddsdio_set_config (noflash)
     rtc_periph (noflash_text)
     rtc_pm (noflash_text)
     rtc_sleep (noflash_text)