stm32mp: stm32prog: use the decimal format by default for offset parsing

Change the default base for offset parsing with simple_strtoull(),
so offset in flashlayout is coded in base 10 by default, even if string
start with '0'. The Octal encoding is not supported. The base 16
is still supported when the '0x' header is detected.

This patch solves an unexpected parsing result when the address,
provided by decimal value is starting by 0, for example 0x4400 = 00017408
is a invalid with current code.

...
P	0x04	fsbl1	Binary	mmc0	00017408	        tf-a.stm32
....

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>