arch/x86/acpigen: wrap _PLD in a package

The ACPI spec has an asl example for _PLD in the form:

Name (_PLD, Package (0x01) { ToPLD (PLD_Revision = 0x2) })

When I ported this to acpigen and diffed the results I noticed that
the binary blob was no longer provided within a package.  The ACPI
spec (section 6.1.8 in version 6.2) defines _PLD as "a variable-length
Package containing a list of Buffers".  This commit changes
acpigen_write_pld to use a package (the one existing caller I found
isn't wrapping the result in a package so it doesn't look like
it was intended for the callers of acpigen_write_pld to be responsible
for using a package.

BUG=none
BRANCH=none
TEST=Verified that after this change a package is use and the result
of acpigen matches what was used in the original asl.

Change-Id: Ie2db63c976100109bfe976553e52565fb2d2d9df
Signed-off-by: Matt Delco <delco@chromium.org>
Reviewed-on: https://review.coreboot.org/c/31162
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/arch/x86/acpigen.c

@@ -1261,7 +1261,9 @@ void acpigen_write_pld(const struct acpi_pld *pld)
 		return;
 
 	acpigen_write_name("_PLD");
+	acpigen_write_package(1);
 	acpigen_write_byte_buffer(buf, ARRAY_SIZE(buf));
+	acpigen_pop_len();
 }
 
 void acpigen_write_dsm(const char *uuid, void (**callbacks)(void *),