at91bootstrap3: bump to v3.6.2

In preparation to add support for the SAMA5D3 Xplained board, this
commit bumps the version of the at91bootstrap3 bootloader to
v3.6.2. While doing this, it also:

 - Allows this bootloader on Cortex-A5 based platforms, since SAMA5D3
   are based on Cortex-A5.

 - Removes a patch that no longer applies, and which has been taken
   into account in at91bootstrap3 upstream.

 - Switches to the upstream Github location as the source.

 - Updates the installation commands to match upstream changes in the
   installation process.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>