realtek: Change Platform defines to depend on CONFIG_RTL83XX

In order for the Platform includes to be available on
all sub-targets, make them dependent on CONFIG_RTL83XX.

Signed-off-by: Birger Koblitz <git@birger-koblitz.de>

target/linux/realtek/files-5.10/arch/mips/rtl838x/Platform

@@ -1,5 +1,5 @@
 #
 # Realtek RTL838x SoCs
 #
-cflags-$(CONFIG_RTL838X)   += -I$(srctree)/arch/mips/include/asm/mach-rtl838x/
-load-$(CONFIG_RTL838X)     += 0xffffffff80000000
+cflags-$(CONFIG_RTL83XX)   += -I$(srctree)/arch/mips/include/asm/mach-rtl838x/
+load-$(CONFIG_RTL83XX)     += 0xffffffff80000000