ARM: asm/pl080.h: Protect against multiple inclusion of header file

<asm/hardware/pl080.h> doesn't have protection to deal with multiple inclusion.
And so we get compilation errors in cases where this file is included more than
once. This patch adds #ifdefs at the top of file to protect it against multiple
inclusions.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>