omap-common: fastboot: extend cpu type for DRA71x rev 2.1

DRA71x processors are reduced pin and software compatible
derivative of DRA72 processors. Extend support for this
revision in "getvar cpu" command.

Signed-off-by: Vishal Mahaveer <vishalm@ti.com>
[praneeth@ti.com: rebase to u-boot master]
Signed-off-by: Praneeth Bajjuri <praneeth@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>