Squashed commit of the following:

    arch/arm/src/lpc54xx:  Adds the LPC54-specific clock configuration logic to the leveraged SD/MMC driver.
    arch/arm/src/lpc54xx:  Leveraged SD/MMC driver builds without error but is still missing clock configuration logic.
    arch/arm/src/lpc54xx:  Bring in LPC43xx SD/MMC driver from https://github.com/Smoothieware/smoothie-nuttx/tree/master/nuttx/arch/arm/src/lpc43xx