acpigen: Add option for reserved bits in Field

Add an option for unused/reserved bits in a Field definition,
allowing for declarations that do not start at bit 0:

Field (UART, AnyAcc, NoLock, Preserve)
{
    , 7,  /* RESERVED */
    BITF, /* Used bit */
}

These just use byte 0 instead of a name.

Change-Id: I86b54685dbdebacb0834173857c9341ea9fa9a46
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46254
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/acpi/acpigen.h

@@ -158,6 +158,10 @@ enum {
 					  .name = X, \
 					  .bits = Y, \
 					}
+#define FIELDLIST_RESERVED(X)		{ .type = RESERVED, \
+					  .name = "", \
+					  .bits = X, \
+					}
 
 #define FIELD_ANYACC			0
 #define FIELD_BYTEACC			1
@@ -174,6 +178,7 @@ enum {
 enum field_type {
 	OFFSET,
 	NAME_STRING,
+	RESERVED,
 	FIELD_TYPE_MAX,
 };