acpi: add STA function to return external variable

This change adds support function to add STA function which returns an
external variable.

Change-Id: I31755a76ee985ee6059289ae194537d531270761
Signed-off-by: Sugnan Prabhu S <sugnan.prabhu.s@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42245
Reviewed-by: Rizwan Qureshi <rizwan.qureshi@intel.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/acpi/acpigen.h

@@ -319,6 +319,7 @@ void acpigen_write_empty_PCT(void);
 void acpigen_write_empty_PTC(void);
 void acpigen_write_PRW(u32 wake, u32 level);
 void acpigen_write_STA(uint8_t status);
+void acpigen_write_STA_ext(const char *namestring);
 void acpigen_write_TPC(const char *gnvs_tpc_limit);
 void acpigen_write_PSS_package(u32 coreFreq, u32 power, u32 transLat,
 			u32 busmLat, u32 control, u32 status);