mfd: vexpress: Allow vexpress-sysreg to self-initialise

The vexpress_sysreg_init() is a core_initcall() already and it can
trigger the early initialisation if a matching node is found. This patch
allows the SoC code to avoid calling vexpress_sysreg_of_early_init()
explicitly.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>