scripts/dtc: Update to upstream version v1.6.0-11-g9d7888cbf19c

Sync with upstream dtc primarily to pickup the I2C bus check fixes. The
interrupt_provider check is noisy, so turn it off for now.

This adds the following commits from upstream:

9d7888cbf19c dtc: Consider one-character strings as strings
8259d59f59de checks: Improve i2c reg property checking
fdabcf2980a4 checks: Remove warning for I2C_OWN_SLAVE_ADDRESS
2478b1652c8d libfdt: add extern "C" for C++
f68bfc2668b2 libfdt: trivial typo fix
7be250b4d059 libfdt: Correct condition for reordering blocks
81e0919a3e21 checks: Add interrupt provider test
85e5d839847a Makefile: when building libfdt only, do not add unneeded deps
b28464a550c5 Fix some potential unaligned accesses in dtc

Signed-off-by: Rob Herring <robh@kernel.org>