smbios: Create a type for smbios_enclosure_type

Add a name to the SMBIOS enclosure type enum and use it as the return
type for smbios_mainboard_enclosure_type.

BUG=b:143701965
TEST=compiles

Change-Id: I816e17f0de2b0c119ddab638e57b0652f53f5b61
Signed-off-by: Mathew King <mathewk@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36516
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: EricR Lai <ericr_lai@compal.corp-partner.google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/x86/smbios.c

@@ -492,7 +492,7 @@ smbios_board_type __weak smbios_mainboard_board_type(void)
 	return SMBIOS_BOARD_TYPE_UNKNOWN;
 }
 
-u8 __weak smbios_mainboard_enclosure_type(void)
+smbios_enclosure_type __weak smbios_mainboard_enclosure_type(void)
 {
 	return CONFIG_SMBIOS_ENCLOSURE_TYPE;
 }