clk: imx6sl: Use BIT(x) to avoid shifting signed 32-bit value by 31 bits

Use readl_relaxed() instead of __raw_readl(), and use BIT(x)
instead of (1 << X) to fix below build warning reported by kernel
test robot:

drivers/clk/imx/clk-imx6sl.c:149:49: warning: Shifting signed 32-bit
value by 31 bits is undefined behaviour [shiftTooManyBitsSigned]
     while (!(__raw_readl(anatop_base + PLL_ARM) & BM_PLL_ARM_LOCK))

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>