ARM: 8821/1: Correct meaning of SCU in HAVE_ARM_SCU help txt

According to the ARM Cortex-A5 and Cortex-A9 Technical Reference
Manuals, SCU stands for "Snoop Control Unit".

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>