ARM64: early_printk: Fix check for CONFIG_ARM64_64K_PAGES

The 'CONFIG_' prefix is not implicit in IS_ENABLED().

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>