mtd: hyperbus: Add hardware dependency to AM654 driver

The hbmc-am654 driver is for the TI AM654, which is an ARM64 SoC, so
don't propose this driver on other architectures unless
build-testing.

Fixes: b07079f1

 ("mtd: hyperbus: Add driver for TI's HyperBus memory controller")
Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>