tpm: atmel: Add explicit include for of.h

With linux/acpi.h (in linux/tpm.h) no longer implicitly including of.h,
add an explicit include of of.h to fix the following errors:

drivers/char/tpm/tpm_atmel.h:50:14: error: implicit declaration of function 'of_find_node_by_name'; did you mean 'bus_find_device_by_name'? [-Werror=implicit-function-declaration]
drivers/char/tpm/tpm_atmel.h:50:12: error: assignment to 'struct device_node *' from 'int' makes pointer from integer without a cast [-Werror=int-conversion]
drivers/char/tpm/tpm_atmel.h:55:14: error: implicit declaration of function 'of_device_is_compatible'; did you mean 'fwnode_device_is_compatible'? [-Werror=implicit-function-declaration]
...

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>