Implement conversions between ErrorCode and u32/u64

Implement TryFrom and From traits for ErrorCode to support conversions
between ErrorCode and u32/u64 types. According to the PSCI
specification, error code values are represented as 32-bit or 64-bit
signed integers. These new trait implementations allow ErrorCode to be
converted to and from u32/u64, making it possible to use the values
directly in registers.

Signed-off-by: Imre Kis <imre.kis@arm.com>
Change-Id: If4b4151760d8d7600d65d1c3d83d317547223457
diff --git a/Cargo.toml b/Cargo.toml
index c2ee1a4..6960b7d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -3,7 +3,7 @@
 
 [package]
 name = "arm-psci"
-version = "0.1.1"
+version = "0.1.2"
 authors = [
     "Balint Dobszay <balint.dobszay@arm.com>",
     "Imre Kis <imre.kis@arm.com>",