ARM: shmobile: r8a7778: Introduce HPBREG_BASE

Replace the hardcoded address of the HPB Register block by a macro.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20201117103022.2136527-2-geert+renesas@glider.be