ARM: socfpga: Fix Documentation errors in scu_registers

According to ARM Cortex-A9 MPCore TRM section 2.2 - SCU registers
Access Control register offset is 0x50.

Signed-off-by: Ben Kalo <ben.h.kalo@gmail.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>