tpm: display warning if using gpio reset with TPM

tpm: display warning if using gpio reset with TPM

Instead of displaying what looks like an error message if a
gpio-reset dt prop is missing for a TPM display a warning that
having a gpio reset on a TPM should not be used for a secure production
device.

TCG TIS spec [1] says:
"The TPM_Init (LRESET#/SPI_RST#) signal MUST be connected to the
platform CPU Reset signal such that it complies with the requirements
specified in section 1.2.7 HOST Platform Reset in the PC Client
Implementation Specification for Conventional BIOS."

The reasoning is that you should not be able to toggle a GPIO and reset
the TPM without resetting the CPU as well because if an attacker can
break into your OS via an OS level security flaw they can then reset the
TPM via GPIO and replay the measurements required to unseal keys
that you have otherwise protected.

Additionally restructure the code for improved readability allowing for
removal of the init label.

Before:
 - board with no reset gpio
u-boot=> tpm init && tpm info
tpm_tis_spi_probe: missing reset GPIO
tpm@1 v2.0: VendorID 0x1114, DeviceID 0x3205, RevisionID 0x01 [open]
 - board with a reset gpio
u-boot=> tpm init && tpm info
tpm@1 v2.0: VendorID 0x1114, DeviceID 0x3205, RevisionID 0x01 [open]

After:
 - board with no reset gpio
u-boot=> tpm init && tpm info
tpm@1 v2.0: VendorID 0x1114, DeviceID 0x3205, RevisionID 0x01 [open]
 - board with a reset gpio
u-boot=> tpm init && tpm info
tpm@1: TPM gpio reset should not be used on secure production devices
tpm@1 v2.0: VendorID 0x1114, DeviceID 0x3205, RevisionID 0x01 [open]

[1] https://trustedcomputinggroup.org/wp-content/uploads/TCG_PCClientTPMInterfaceSpecification_TIS__1-3_27_03212013.pdf

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>