rpi3: Move rng driver to drivers ...

To allow sharing the driver between the RPi3 and RPi4, move the random
number generator driver into the generic driver directory.

Change-Id: Iae94d7cb22c6bce3af9bff709d76d4caf87b14d1
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

rpi3: Move VC mailbox driver into generic drivers directory ...

To allow sharing the driver between the RPi3 and RPi4, move the mailbox
driver into the generic driver directory.

Change-Id: I463e49acf82b02bf004f3d56482b7791f3020bc0
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

rpi3: sdhost: SDHost driver improvement ...

This commit improves the SDHost driver for RPi3 as following:
 * Unblock MMC_CMD(17). Using MMC_CMD(17) is more efficient on
   block reading.
 * In some low probability that SEND_OP_COND might results CRC7
   error. We can consider that the command runs correctly. We don't
   need to retry this command so removing the code for retry.
 * Using MMC_BUS_WIDTH_1 as MMC default value to improve the stability.
 * Increase the clock to 50Mhz in data mode to speed up the io.
 * Change the pull resistors configuration to gain more stability.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>

rpi3: Add SDHost driver ...

This commit adds SDHost driver for RaspberryPi3. SDHost driver uses the
GPIO driver to connect the SDCard and SDHost. By using this driver it is
able to read/write raw blocks on SDCard.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>

rpi3: Add GPIO driver ...

This commit adds GPIO driver for RaspberryPi3. The GPIO driver for RPi3
also provides the way to do pinmux selections.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>