riscv: sbi: Add SUSP tests

Introduce tests for SBI system suspend. The basic test makes
sure it works and other tests make sure it fails as expected
with invalid entry criteria.

To test on QEMU or hardware the firmware needs to support system
suspend. For QEMU, OpenSBI can be told to enable its system
suspend test mode by creating a new DTB which has

    opensbi-config {
        compatible = "opensbi,config";
        system-suspend-test;
    };

added to the 'chosen' node. Then, run with '-dtb susp.dtb'.

Signed-off-by: Andrew Jones <andrew.jones@linux.dev>