bcm27xx: disable duplicate sdhost driver

bcm27xx: disable duplicate sdhost driver

Enabling both CONFIG_MMC_BCM2835 and CONFIG_MMC_BCM2835_SDHOST causes this
error in dmesg:
Error: Driver 'sdhost-bcm2835' is already registered, aborting...

Disabling CONFIG_MMC_BCM2835 and leaving CONFIG_MMC_BCM2835_SDHOST enabled
avoids this error.

Build-tested: bcm2711/RPi4B
Run-tested: bcm2711/RPi4B

Signed-off-by: John Audia <[email protected]>
[Disable driver for all subtargets, refresh configs, tweak description]
Signed-off-by: Álvaro Fernández Rojas <[email protected]>