recipes-bsp: u-boot-ti-mainline: Update to v2021.04 release
recipes-bsp: u-boot-ti-mainline: Update to v2021.04 release

Update u-boot recipe to be on the bleeding upstream tag.

While at it, also update the u-boot source url to point to the new
server as per [1].

While it may be redundant, lets document the usage of URI and
PROTOCOL for those, who (cough), may get tripped otherwise.

[1] https://source.denx.de/u-boot/u-boot/-/commit/a3bbd0b912f9afff745247fe9641961875322926

Signed-off-by: Nishanth Menon <50c3c575ce2bb12c45e20736ce59e86fe81915b4@ti.com>
Reviewed-by: Denys Dmytriyenko <d29de71aea38aad3a87d486929cb0aad173ae612@konsulko.com>

rcn-ee/meta-ti,rcn-ee/meta-ti,rcn-ee/meta-ti,rcn-ee/meta-ti