riscv: use uint8_t for register_t

`register_t` only needs to be able to index into the TCB user context
array, which has 35 entries. Therefore `uint8_t` is sufficient.

Using the smallest possible type for `register_t` helps with binary
verification. This shrinks static read-only data, which in turn reduces
the complexity of binary verification proof search.

Signed-off-by: Matthew Brecknell <Matthew.Brecknell@data61.csiro.au>

include/arch/riscv/arch/machine/registerset.h

@@ -72,7 +72,7 @@ enum _register {
     n_contextRegisters
 };
 
-typedef word_t register_t;
+typedef uint8_t register_t;
 
 enum messageSizes {
     n_msgRegisters = 4,