acpi/acpigen: add acpigen_write_BBN to generate base bus number method

Introduce acpigen_write_BBN to generate the ACPI method object that
returns the base bus number for a PCI(e) host bridge. When called, the
base_bus_number argument must be the first PCI bus number that got
assigned to the corresponding host bridge.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: Ib67bf42b9c77c262d8a02d8f28ac5cb8482136b9
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74991
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <inforichland@gmail.com>

src/acpi/acpigen.c

@@ -821,6 +821,17 @@ void acpigen_write_STA_ext(const char *namestring)
 	acpigen_pop_len();
 }
 
+void acpigen_write_BBN(uint8_t base_bus_number)
+{
+	/*
+	 * Method (_BBN, 0, NotSerialized) { Return (status) }
+	 */
+	acpigen_write_method("_BBN", 0);
+	acpigen_emit_byte(RETURN_OP);
+	acpigen_write_byte(base_bus_number);
+	acpigen_pop_len();
+}
+
 void acpigen_write_LPI_package(u64 level, const struct acpi_lpi_state *states, u16 nentries)
 {
 	/*

src/include/acpi/acpigen.h

@@ -435,6 +435,7 @@ void acpigen_write_PTC(uint8_t duty_width, uint8_t duty_offset, uint16_t p_cnt);
 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_BBN(uint8_t base_bus_number);
 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);