Sept 5 (Reuters) - SoftBank Group's (9984.T) Arm is seeking a valuation of more than $52 billion in its initial public offering, the chip designer said on Tuesday as it begins marketing for the biggest U.S. stock market flotation of the year. SoftBank is offering 95.5 million American depository shares of the United Kingdom-based company for $47 to $51 apiece and is looking to raise up to $4.87 billion at the top of the range, Arm said in a regulatory filing. The valuation that Arm is chasing now represents a climb-down from the $64 billion valuation at which SoftBank last...