arm: mvebu: a38x: serdes: Remove duplicate macro SOC_CTRL_REG

SoC Control 1 Register (offset 0x18204) is already defined by macro
SOC_CONTROL_REG1.

Use macro SOC_CONTROL_REG1 instead of macro SOC_CTRL_REG in ctrl_pex.c
code and remove the other definition.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>