samd/boards: Add Mini SAM M4 board configuration.

ports/samd/boards/MINISAM_M4/mpconfigboard.h

+#define MICROPY_HW_BOARD_NAME "Mini SAM M4"
+#define MICROPY_HW_MCU_NAME   "SAMD51G19A"
+
+#define MICROPY_FLOAT_IMPL              (MICROPY_FLOAT_IMPL_FLOAT)
+#define MICROPY_PY_BUILTINS_COMPLEX     (0)
+#define MICROPY_PY_MATH                 (0)
+#define MICROPY_PY_CMATH                (0)

ports/samd/boards/MINISAM_M4/mpconfigboard.mk

+# https://www.minifigboards.com/mini-sam-m4/mini-sam-m4-hardware/
+MCU_SERIES = SAMD51
+CMSIS_MCU = SAMD51G19A
+LD_FILES = boards/samd51g19a.ld sections.ld
+TEXT0 = 0x4000