dt-bindings: tpm: Add compatible string atmel,attpm20p

Commit 4f2a348a ("arm64: dts: imx8mm-venice-gw73xx: add TPM device")
added a devicetree node for the Trusted Platform Module on certain
Gateworks boards.

The commit only used the generic "tcg,tpm_tis-spi" compatible string,
but public documentation shows that the chip is an ATTPM20P from Atmel
(nowadays Microchip):
https://trac.gateworks.com/wiki/tpm

Add the chip to the supported compatible strings of the TPM TIS SPI
schema.

For reference, a datasheet is available at:
https://ww1.microchip.com/downloads/en/DeviceDoc/ATTPM20P-Trusted-Platform-Module-TPM-2.0-SPI-Interface-Summary-Data-Sheet-DS40002082A.pdf



Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Cc: Tim Harvey <tharvey@gateworks.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>