u-boot-at91: Bump to linux4sam-2021.04-rc3
u-boot-at91: Bump to linux4sam-2021.04-rc3

Signed-off-by: Codrin Ciubotariu <97034fe81dd68ee5e1fa3aa6216f6373b034daef@microchip.com>

linux4sam/meta-atmel,linux4sam/meta-atmel,linux4sam/meta-atmel,linux4sam/meta-atmel,linux4sam/meta-atmel