drivers: serial: Kconfig.uart_sam: Enable support to SAMV71

SAMV71 uses same driver of SAME70 and this enables the feature. Since
driver is shared with other devices, it must be enabled conditionally
based on the number of instances. This avoid show invalid options for
devices with lower port count like SAM4S/SAM4E.

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>