MX51: removed warnings for the mx51evk

The patch removes warnings at compile time and provides
some cleanup code:
- Removed comment on NAND (not yet supported) from lowlevel_init.S
- Removed NFMS bit definition from imx-regs.h
  The bit is only related to MX.25/35 and can lead to confusion
- Moved is_soc_rev() to soc specific code (removed from mx51evk.c)

Signed-off-by: Stefano Babic <sbabic@denx.de>
diff --git a/include/asm-arm/arch-mx51/clock.h b/include/asm-arm/arch-mx51/clock.h
index 449a191..1f8a537 100644
--- a/include/asm-arm/arch-mx51/clock.h
+++ b/include/asm-arm/arch-mx51/clock.h
@@ -23,9 +23,21 @@
 
 #ifndef __ASM_ARCH_CLOCK_H
 #define __ASM_ARCH_CLOCK_H
+
+enum mxc_clock {
+	MXC_ARM_CLK = 0,
+	MXC_AHB_CLK,
+	MXC_IPG_CLK,
+	MXC_IPG_PERCLK,
+	MXC_UART_CLK,
+	MXC_CSPI_CLK,
+	MXC_FEC_CLK,
+};
+
 unsigned int imx_decode_pll(unsigned int pll, unsigned int f_ref);
 
-ulong imx_get_uartclk(void);
-ulong imx_get_fecclk(void);
+u32 imx_get_uartclk(void);
+u32 imx_get_fecclk(void);
+unsigned int mxc_get_clock(enum mxc_clock clk);
 
 #endif /* __ASM_ARCH_CLOCK_H */