meta-aspeed: add Tan as owner for U-Boot

As announced on Discord [0], I joined the maintenance team for
openbmc/u-boot and will help maintaining the tree.

Link: [0] https://discord.com/channels/775381525260664832/775694683589574659/1541972097117720687
Change-Id: I298dce057e23b9716f5c9551640905460a88c053
Signed-off-by: Tan Siewert <tan.siewert@9elements.com>

meta-aspeed/OWNERS

diff --git a/meta-aspeed/OWNERS b/meta-aspeed/OWNERS
index 16b5f65..5530ed0 100644
--- a/meta-aspeed/OWNERS
+++ b/meta-aspeed/OWNERS
@@ -1,3 +1,8 @@
 owners:
 - joel@jms.id.au
 - andrew@codeconstruct.com.au
+
+matchers:
+- partial_regex: recipes-bsp/u-boot
+  owners:
+  - tan@siewert.io