MAINTAINERS: Add stm32 and stm regexp for ARM STM STM32MP platform

Add files and directories regex "stm32" and "stm" in "ARM STM STM32MP"
platform to avoid missing files or drivers supported by the
STMicroelectronics series STM32MP15x.

This patch adds the rules already used in Linux kernel for
ARM/STM32 ARCHITECTURE.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index fb9ba37..1e796cb 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -443,6 +443,8 @@
 F:	include/dt-bindings/reset/stm32mp1-resets.h
 F:	include/stm32_rcc.h
 F:	tools/stm32image.c
+N:	stm
+N:	stm32
 
 
 ARM STM STV0991