soc: ep93xx: drop reference to removed EP93XX_SOC_COMMON config

Commit 6eab0ce6 ("soc: Add SoC driver for Cirrus ep93xx") adds the
config EP93XX_SOC referring to the config EP93XX_SOC_COMMON.

Within the same patch series of the commit above, the commit 046322f1
("ARM: ep93xx: DT for the Cirrus ep93xx SoC platforms") then removes the
config EP93XX_SOC_COMMON. With that the reference to this config is
obsolete.

Simplify the expression in the EP93XX_SOC config definition.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Reviewed-by: Nikita Shubin <nikita.shubin@maquefel.me>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>