rpi: use board_early_init_r to init PCI/USB and detect boot device

rpi: use board_early_init_r to init PCI/USB and detect boot device

Move initialization of the PCI and the USB subsystems from the 'preboot'
command to the board's early_init_r() function and then try to detect a
boot device by accessing the MMC0 device. If it works, then set boot
device interface to 'mmc', if not, use 'usb'.

This allows the environment variales to be loaded from the proper
storage interface.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Change-Id: Idf06ba827167479510206587308875c8eedce892