cmd: optee_rpmb: build cmd for sandbox

Support CMD_OPTEE_RPMB for SANDBOX configurations.
Test:

$ ./u-boot -d arch/sandbox/dts/test.dtb
...
=> optee_rpmb write_pvalue test_variable test_value
Wrote 11 bytes
=> optee_rpmb read_pvalue test_variable 11
Read 11 bytes, value = test_value

Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>