hw/registerfields.h: Add 8bit and 16bit register macros

hw/registerfields.h: Add 8bit and 16bit register macros

Some RX peripheral use 8bit and 16bit registers.
Add the 8bit and 16bit APIs.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Yoshinori Sato <ysato@users.sourceforge.jp>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200224141923.82118-4-ysato@users.sourceforge.jp>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>