cmd: cpu: fix NULL cpu feature prints

cmd: cpu: fix NULL cpu feature prints

Commit 740d5d3 added two new features but only one feature name,
which results in NULL prints when device_id feature is selected.

Before:
	HG556a # cpu detail
	 -1: cpu@0	BCM6358A1
		ID = 0, freq = 300 MHz: L1 cache, MMU, NULL
		Device ID 0x2a010
	 -1: cpu@1	BCM6358A1
		ID = 1, freq = 300 MHz: L1 cache, MMU, NULL
		Device ID 0x2a010
After:
	HG556a # cpu detail
	 -1: cpu@0	BCM6358A1
		ID = 0, freq = 300 MHz: L1 cache, MMU, Device ID
		Device ID 0x2a010
	 -1: cpu@1	BCM6358A1
		ID = 1, freq = 300 MHz: L1 cache, MMU, Device ID
		Device ID 0x2a010

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>