soc: xilinx: vcu: add missing register NUM_CORE

The H.264/H.265 Video Codec Unit v1.2 documentation describes this
register as follows:

	Number of encoders core used for the provided configuration

This is required for configuring the VCU encoder buffer.
Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Reviewed-by: Hyun Kwon <hyun.kwon@xilinx.com>
Link: https://lore.kernel.org/r/20201109134818.4159342-5-m.tretter@pengutronix.deSigned-off-by: Michal Simek <michal.simek@xilinx.com>

include/linux/mfd/syscon/xlnx-vcu.h

@@ -32,6 +32,7 @@
 #define VCU_BUFFER_B_FRAME		0x5c
 #define VCU_WPP_EN			0x60
 #define VCU_PLL_CLK_DEC			0x64
+#define VCU_NUM_CORE			0x6c
 #define VCU_GASKET_INIT			0x74
 #define VCU_GASKET_VALUE		0x03