ARM: dts: at91: sama5d3_uart: fix reg sizes to match documentation

The new size (0x100) also matches the size given in sama5d3.dtsi

Documentation reference: section 43.6 "Universal Asynchronous
Receiver Transmitter (UART) User Interface", table 43-4 "Register
Mapping" in [1].

[1] Atmel-11121F-ATARM-SAMA5D3-Series-Datasheet_02-Feb-16

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>