ARM: dts: at91: sama5d2/trivial: fix letter case for etm hex address

ARM: dts: at91: sama5d2/trivial: fix letter case for etm hex address

Fix the etm node hex address to lower case for matching regexp
specification and removing the additional warning that looks like:

arch/arm/boot/dts/at91-sama5d2_ptc_ek.dt.yaml: /: 'etm@73C000' does not
match any of the regexes: '@(0|[1-9a-f][0-9a-f]*)$', '^[^@]+$',
'pinctrl-[0-9]+'

Reported-by: Arnd Bergmann <arnd@kernel.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>