util/scripts/show_platforms.sh: Fix reStructuredText table output

util/scripts/show_platforms.sh: Fix reStructuredText table output

reStructuredText grid tables require row separators otherwise the rows
get concatenated into a single cell for each column.

Representative output of previous behavior:
```eval_rst
+-------------------------+-------------------+------------+----------+
| Vendor/Board            | Processor         | Date added | Brd type |
+=========================+===================+============+==========+
| 51nb/x210               | INTEL_KABYLAKE    | 2020-03-16 | laptop   |
| acer/aspire_vn7_572g    | INTEL_SKYLAKE     | 2022-01-28 | laptop   |
| acer/g43t-am3           | INTEL_X4X         | 2020-09-28 | desktop  |
+-------------------------+-------------------+------------+----------+
```

Representative output of corrected behavior:
```eval_rst
+-------------------------+-------------------+------------+----------+
| Vendor/Board            | Processor         | Date added | Brd type |
+=========================+===================+============+==========+
| 51nb/x210               | INTEL_KABYLAKE    | 2020-03-16 | laptop   |
+-------------------------+-------------------+------------+----------+
| acer/aspire_vn7_572g    | INTEL_SKYLAKE     | 2022-01-28 | laptop   |
+-------------------------+-------------------+------------+----------+
| acer/g43t-am3           | INTEL_X4X         | 2020-09-28 | desktop  |
+-------------------------+-------------------+------------+----------+
```

Change-Id: I83be58dd2c34c65ae2c65cf2bd98330936fb6f6a
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/72624
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>