soc: xtensa: espressif_esp32: add SPIRAM heap offset and size

The existing heap starts at the beginning of SPIRAM which is also occupied
by the WiFi stack if CONFIG_ESP32_WIFI_NET_ALLOC_SPIRAM is enabled.  This
results in memory corruption.

Update the linker description to add a _spiram_heap_start symbol and add a
KConfig size option to allow the SPIRAM stack to be correctly located and
sized.  Requires matching commit in hal_espressif.

Fixes: 61359

Signed-off-by: Eric Holmberg <eric.holmberg@northriversystems.co.nz>