mtd: hyperbus: Kconfig: Fix HBMC_AM654 dependencies

On x86_64, when CONFIG_OF is not disabled:

WARNING: unmet direct dependencies detected for MUX_MMIO
  Depends on [n]: MULTIPLEXER [=y] && (OF [=n] || COMPILE_TEST [=n])
  Selected by [y]:
  - HBMC_AM654 [=y] && MTD [=y] && MTD_HYPERBUS [=y]

due to
config HBMC_AM654
	tristate "HyperBus controller driver for AM65x SoC"
	select MULTIPLEXER
	select MUX_MMIO

Fix this by making HBMC_AM654 imply MUX_MMIO instead of select so
that dependencies are taken care of. MUX_MMIO is optional for
functioning of driver.

Fixes: b07079f1

 ("mtd: hyperbus: Add driver for TI's HyperBus memory controller")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>