kconfig: add missing SPI TPM CS config definition

To fully define TPM attachment to a SPI interface both bus and CS
(chip select) settings are required. Add the missing CS configuration
option.

BRANCH=none
BUG=chrome-os-partner:50645
TEST=with the rest of the patches applied it is possible to compile in
     and run TPM2 SPI driver.

Change-Id: If297df8e5b9526f156ed1414eb9db317d6af5b33
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/353913
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/15299
Tested-by: build bot (Jenkins)
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@googlemail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/drivers/spi/tpm/Kconfig

@@ -6,3 +6,8 @@ config DRIVER_TPM_SPI_BUS
 	hex "SPI bus TPM chip is connected to"
 	default 0
 	depends on SPI_TPM
+
+config DRIVER_TPM_SPI_CHIP
+	int "Chip Select of the TPM chip on its SPI bus"
+	default 0
+	depends on SPI_TPM