rockchip: Move config SYS_MALLOC_LEN to Kconfig

Use Kconfig for option SYS_MALLOC_LEN and default to 0x2000000.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/Kconfig b/Kconfig
index d2eb744..d5538dd 100644
--- a/Kconfig
+++ b/Kconfig
@@ -156,7 +156,8 @@
 
 config SYS_MALLOC_LEN
 	hex "Define memory for Dynamic allocation"
-	depends on ARCH_ZYNQ || ARCH_VERSAL || ARCH_STM32MP
+	depends on ARCH_ZYNQ || ARCH_VERSAL || ARCH_STM32MP || ARCH_ROCKCHIP
+	default 0x2000000 if ARCH_ROCKCHIP
 	help
 	  This defines memory to be allocated for Dynamic allocation
 	  TODO: Use for other architectures