OvmfPkg: Add RAM disk support

OvmfPkg: Add RAM disk support

Currently booting off of a RAM disk is not supported by
IntelFrameWorkModulePkg BDS, however on systems without writable
disks, the RAM disk can be made useful when loading raw HDD images
into it -- specially the ones with a FAT32 partition on which files
can be natively accessed by system firmware.

This patch adds RamDiskDxe driver by default in OVMF platform.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Paulo Alcantara <paulo.alc.cavalcanti@hp.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>