Documentation: tpm: add the IBM Virtual TPM device tree binding documentation

* Device Tree Bindings for IBM Virtual Trusted Platform Module(vtpm)

Required properties:

- compatible            : property name that conveys the platform architecture

                          identifiers, as 'IBM,vtpm'

- device_type           : specifies type of virtual device

- interrupts            : property specifying the interrupt source number and

                          sense code associated with this virtual I/O Adapters

- ibm,my-drc-index      : integer index for the connector between the device

                          and its parent - present only if Dynamic

                          Reconfiguration(DR) Connector is enabled

- ibm,#dma-address-cells: specifies the number of cells that are used to

                          encode the physical address field of dma-window

                          properties

- ibm,#dma-size-cells   : specifies the number of cells that are used to

                          encode the size field of dma-window properties

- ibm,my-dma-window     : specifies DMA window associated with this virtual

                          IOA

- ibm,loc-code          : specifies the unique and persistent location code

                          associated with this virtual I/O Adapters

- linux,sml-base        : 64-bit base address of the reserved memory allocated

                          for the firmware event log

- linux,sml-size        : size of the memory allocated for the firmware event log

Example (IBM Virtual Trusted Platform Module)

---------------------------------------------

                vtpm@30000003 {

                        ibm,#dma-size-cells = <0x2>;

                        compatible = "IBM,vtpm";

                        device_type = "IBM,vtpm";

                        ibm,my-drc-index = <0x30000003>;

                        ibm,#dma-address-cells = <0x2>;

                        linux,sml-base = <0xc60e 0x0>;

                        interrupts = <0xa0003 0x0>;

                        ibm,my-dma-window = <0x10000003 0x0 0x0 0x0 0x10000000>;

                        ibm,loc-code = "U8286.41A.10082DV-V3-C3";

                        reg = <0x30000003>;

                        linux,sml-size = <0xbce10200>;

                };