cmd: mtd: Enable speed benchmarking

cmd: mtd: Enable speed benchmarking

Linux features a flash_speed speed test from the mtd-utils suite, U-Boot
does not. Benchmarks are useful for speed improvement developments as
well as troubleshooting or regression testing sometimes.

Enable a benchmark option to enable this feature.

Example of output on a Nuvoton platform:

MA35D1> mtd read nor0 0x81000000 0 0x10000
Reading 65536 byte(s) at offset 0x00000000
MA35D1> mtd read.benchmark nor0 0x81000000 0 0x10000
Reading 65536 byte(s) at offset 0x00000000
Read speed: 3752kiB/s

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>