m68knommu: include SDHC support only when hardware has it

The mere fact that the kernel has the MMC subsystem enabled (CONFIG_MMC
enabled) does not mean that the underlying hardware platform has the
SDHC hardware present. Within the ColdFire hardware defines that is
signified by MCFSDHC_BASE being defined with an address.

The platform data for the ColdFire parts is including the SDHC hardware
if CONFIG_MMC is enabled, instead of MCFSDHC_BASE. This means that if
you are compiling for a ColdFire target that does not support SDHC but
enable CONFIG_MMC you will fail to compile with errors like this:

    arch/m68k/coldfire/device.c:565:12: error: ‘MCFSDHC_BASE’ undeclared here (not in a function)
       .start = MCFSDHC_BASE,
            ^
    arch/m68k/coldfire/device.c:566:25: error: ‘MCFSDHC_SIZE’ undeclared here (not in a function)
       .end = MCFSDHC_BASE + MCFSDHC_SIZE - 1,
                         ^
    arch/m68k/coldfire/device.c:569:12: error: ‘MCF_IRQ_SDHC’ undeclared here (not in a function)
       .start = MCF_IRQ_SDHC,
            ^

Make the SDHC platform support depend on MCFSDHC_BASE, that is only
include it if the specific ColdFire SoC has that hardware module.

Fixes: 991f5c4d ("m68k: mcf5441x: add support for esdhc mmc controller")
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Angelo Dureghello <angelo.dureghello@timesys.com>
Tested-by: Angelo Dureghello <angelo.dureghello@timesys.com>