Fix ROM / RAM size on atsamd51j20

[sago35]

In this PR, I'll correct the linker script.

ROM size of the atsamd51j20a is 1024KB, not 512KB.
RAM size is 256KB, not 192KB.

[sago35]

The difference from atsamd51.ld is the following.

$ diff -u targets/atsamd51.ld targets/atsamd51j20a.ld
--- targets/atsamd51.ld Sun Feb 23 23:46:31 2020
+++ targets/atsamd51j20a.ld     Tue May 05 08:00:40 2020
@@ -1,8 +1,8 @@

 MEMORY
 {
-    FLASH_TEXT (rw) : ORIGIN = 0x00000000+0x4000, LENGTH = 0x00080000-0x4000  /* First 16KB used by bootloader */
-    RAM (xrw)       : ORIGIN = 0x20000000, LENGTH = 0x00030000
+    FLASH_TEXT (rw) : ORIGIN = 0x00000000+0x4000, LENGTH = 0x00100000-0x4000  /* First 16KB used by bootloader */
+    RAM (xrw)       : ORIGIN = 0x20000000, LENGTH = 0x00040000
 }

 _stack_size = 4K;

[sago35]

ATSAMDJ20A is only used in pyportal.json, so it only affects pyportal.

$ git rev-parse HEAD
5561b2258b452e0b0fe72b5dea51506e119f8dfa

$ grep inherit.*samd51 targets/*
targets/feather-m4.json:    "inherits": ["atsamd51j19a"],
targets/itsybitsy-m4.json:    "inherits": ["atsamd51g19a"],
targets/metro-m4-airlift.json:    "inherits": ["atsamd51j19a"],
targets/pybadge.json:    "inherits": ["atsamd51j19a"],
targets/pyportal.json:    "inherits": ["atsamd51j20a"],

$ grep amd51.*ld targets/*
targets/atsamd51g19a.json:      "linkerscript": "targets/atsamd51.ld",
targets/atsamd51j19a.json:      "linkerscript": "targets/atsamd51.ld",
targets/atsamd51j20a.json:      "linkerscript": "targets/atsamd51j20a.ld",

[sago35]

The data sheet can be found below.

P.17 ～ P.18
Configuration Summary
http://ww1.microchip.com/downloads/en/DeviceDoc/60001507E.pdf

Device	ROM	RAM
atsamd51j19	512KB	192KB
atsamd51g19	512KB	192KB
atsamd51j20	1024KB	256KB

[aykevl]

Thank you, looks good to me!